AS£ H. *■-■J- # s#: .-;-.- •>jj>- mm: y^r )'j.O'UOO£)'Cgt XQ uQV,-O'QQQQrCQr0Q'u~s$ v Surgeon General's Office ^, la 6* /£ [Plate VIII—Star Fishes 1 The star fish is also gradually transformed from its outlines in Plate IV, into the perfect animal, (Plate VIII.) It now becomes an important point to be able to ascertain to what peculiar forms of Sea-urchin those embryos belong,as we have among the living ones some with the flattened disks, oth- ers with a spherical form, and others with more prominent elongated forms. Let us see what sort of living forms we have among Sea-urchins. There are some in which large plates alternate with very small ones (Plate XIII fig. A) which are called Ci- daris. There are others, in which the plates are more numerous, in which the rows of holes are broader (Fig. B.) and in which the spines are small, Echinus. There are others in which we have plates still more numerous, (Fig. C ) the body more coni- cal, the rows of holes being still larger, and tha spines reduced almost to little heads, Holopneustes. On the shores of the Northern Sea,'where the above described larva? of Sea Urchins were observed, there is no Echinoderm found belonging to the ge- nus Cidaris. Nevertheless, you will notice that that young Sea-urchin of Plate XI fig. G. has re- markably large spines, equalling nearlythe whole diameter of the animal, although in its perfect con- dition it will have proportionally small ones. From that very fact we can conclude that the Cidaris stands lower than the Echinus; though it is usu- ally considered a more elegant and higher form.— This conclusion must be granted at once, when we consider the great disproportion in the size of the spines in Cidaris. and the large plates for the spines resembling the embryonic form of the Echinus that the genus Cidaris ranks lower than Echinus. In Holopneustes, (Plate XIII, fig. C) in which the rows of holes are wider still than in Echinus LECTURES ON EMBRYOLOGY. 23 approximating thus to,Holothuria;, and the body more elongated. We have really a still higher de gree of developement. In the general classification of these animals, I showed that the tubular form is the highest, as is seen among the Holothuria; (Plates XIV and V). [Plate XIV—Holothuria.] TPlate XV—Fasstt, <~VlNOTT>8 ] iPLAlH V—UuLOTHUlilA. | I might have shown these animals to remind you of what are the species on these shores. Here is the common Five finger (Plate VIII, fig. A), and here is the common Sea-urchin (Plate XIII, fig B) a spherical body covered with spines, which may assist us in comparing, better than simple dia- grams, these animals with their embryonic states, as illustrated before. [Plate I—Comatit.a.I L„-t us now also compare those embryonic forms with the fossils of different geological epochs. How the young Comatula (Plate I) casts off the stem, I have already mentioned ; but if we consider its embryonic form, it will compare most remarkably with the fossils figured here (Plate XV). In other instances, however, the fossil Crinoids do not even resemble the young of those of our present epoch, but belong altogether to peculiar types, as figured here (Plate XVII). I have been able to bring 'iere a natural speci wen of one of these lily-like animals, in a most pence:, sctic ol preservation,resting upon us a.cu), which is composed of innumerable plates articu- lating together. It is a Tentacrinus, from Wurt- emberg, in Germany. The principal portion of the animal, which is called its crown, divides into five distinct rays, which are flattened down upon the slab of stone upon which it rests, but so well pre- served that every one of the ramifications can be distingushed, and the connexion of these branches upon the crown below are very distinct. (The Prof, here showed a most splendid fossil, which ex- cited great interest among the audience. Those in- terested in this branch of natural history will find the subject carefully investigated in Agassiz and Gould's Principles of Zoology.) I doubt whether there is another specimen so perfect as this, and I would invite you after the lecture to pass by it and observe it. The number of joints which allow the animal to move and expand is enormous. One hundred and fifty thousand have been computed in one of them by Dr. Buckland : in others, the number of joints are fewer (Plate XV.), the crown remaining more closed and the rays not dividing so extensively 24 PROF. AGASSIZ'S These animals which were extremely numerous in former geological ages, agree in the mode of growth of their plates, with the yoang of that star- fish called Comarala, as it has been observed by Thompson. This diagram (Plate I, fig A.) seems to represent a large animal but it is only highly magnified, the natural size of it being only half an inch long. Nevertheless we distinguish in it the articulated stem with joints. We have the crown above with its solid plates. We have the dividing arm arising from it. We have the surrounding tentacles contributing to seize its prey and bring it to the mouth, and the movable tentacles or Back- ers along the inner side of the branched rays, whieh this animal moves as the others use their suckers. In addition to these, there are gradually j more tentacles coming out, and the body grows larger, till it is freed from its stem in its perfect condition. The star-fishes which do not rest upon a stem and whieh do not branch, resemble less | those fossils than the types of them in which the rays are more numerous and in which the rays branch (Plate I, fig. B.) But even in common star- i fishes in their earliest condition (Plates IV, and X, j fig. A.), we have an arrangement of the solid part* waleh resemble more closely the arrangement of the Solid parts of Crinoids (Plate X, fig. B.) than the arrangements of parts in the full grown star- fish (Plate VIII, fig. A.) Compare the solid plate in the young starfish with the solid plates of the fnUgrown animal. In the young we have a star in in which five large plates seem to alternate with five j others, ten of them forming the principle mass of the body. [Plate X—Comparison of the Calcareous Net Works of Star Fishes, with the Solid Plates of Crinoids.] If we take the Pentacrinus (Plate XV, fig. A) we observe above the stem a crown, in which five large plates, forming the cup, alternate with five smaller ones. In Apiocrinus, the larger plates constitute a hollow cup and above them alternating with them, there are others (Plate XV, fig. B) upon which the branching arms rest. In Encrinus crown and arms are not so widely separated and seem to form still an undivided cavity, as in the genera of Plate XVII. (Plate VII, fig. D). Everywhere the same arrangement exists, so that on a diagram the same drawing would answer for the crinoids and the common star-fishes indiscriminately. Here (Plate X, fig. A) is the central network of the common star-fish, corresponding to the (stem of Pentacrinus, here the five plates which surround the mouth, and those alternating with them, will form the five rays, and so on with successive little plates in all the genera. [Plate VII—Star Fishes—Crinoids.] In Plate X , fig. B, we have the corresponding parts in a diagram of a crinoid, answering precise- ly in position, number, and mode of growth, the solid frame of the starfishes. In all we find a plan which is uniform* whether we observe such ani- mals in which the young are provided with a stem, or those in which the stem does not appear at all. Even if we go back to the young echini, which seem to differ so much from the starfishes, we have an identical number of primitive suck- ers, namely, five,- (Plate XI, fig. F) which do not giro rise to a pentagonal body until the flattened dise assumes a more spherical form (Fig. G). So that there is a most intimate agreement between the different growths of the embryo. All these data upon the embryonic changes of Echinoderms are very fragmentary, as I have al- ready remarked; nevertheless, with these incom- plete series of observations, it can be shown, as I think I have done, that these embryonic forms agree intimately with those which occupy a higher rank in the class, and that they resemble also the form of those which existed in former geological ages. Would these data afford the means of now in- LECTURES ON EMBRYOLOGY. 25 troducing a natural classification among these an- imals? is a further question which lays in my plan; as these embryonic investigations were trac- ed from the beginning with reference to the classi- fication of the animal kingdom in relation to the order of all types, when compared with the chang- es which embryos undergo. Among Echinoderms the investigation of struc- ture has already settled the classification to this extent,that they have been divided into three fam- ailies,Holothurians,the tubular ones, Plates XIV & V, & V; Echini,the spherical ones, Plate XIII; and the Asterians, (Plate VIII,) the star-shaped ones: but from this general arrangement there is still a con- siderable distance to the perfect fixation of the or- der of succession of genera in all their details. The various arrangements which have been proposed have been influenced by the various states of our knowledge. The improvements in the classifica- tion of Echinoderms have been greatly advanced by the knowledge of the Crinoids, which are universally placed in the lowest rank among those animals, from their resemblance to Polyps. When their structure was ascertained,the knowledge thus acquired, did not modify the position which was assigned to them when not yet sufficiently known. The knowledge of the change in the growth of one Crinoid, the Comatula, has indeed influenced more the classification than the knowledge of their struc- ture. The free star-fishes are placed next to the Echini and above all the Holothuria?. Among Ech- ini we have some in which the mouth is central and the alimentary canal ends on the margin ; and there are others in which the alimentary canal ends on the two extremities of the body, as seen here, (Plate VI fig. B.) thus forming a transition to the worm-like form, they indeed begin to be re- lated to the Holothuriae (Plate XIV) and will rank higher. [Plate VI—Sea Urchins ] Structure and embryonic growth have satisfied u8 thus far. But why should we not venture to go fur- ther,and make use of the order of succession of these types, in order to ascertain all their relations ? The Crinoids whieh have been described as fossils, are exceedingly numerous. Here are figured several forms, to which I have not yet alluded. Plate XVII, fig. C, is a genus called Caryoirinus. Here is another, which occurs also in old strata, (Fig. B) called Pentremites; and here (Fig. D) one which oc- curs in deposites of the coal period, called Echino- crinus. In Plate XVII, fig. C, we have a spherieal body, like an Echinus,with a stem as in Crinoids, but the plates are not yet ranged in regular rows (Fig. C), but alternate irregularly ; there are not yet rows for the pores distinctly circumscribed, but only at irregular intervals, and few of them. This form, as also the Sphoronites are the most primitive Cri- noids, and they correspond somewhat in structure to the earliest condition which we observe in Echi- ni, and which we observe also in the youngest stage of the star fish. Here is one (Plate XVII, fig B) in which we have a mere star-fish-like form; the sphere is in Its full condition of development; and here we have one which would seem to be a common sea-urchin, (Fig. D.) But on comparing both (Plate XIII, fig. C) they are found widely different. In Echinus (Plate XIII, fig. C) there are two rows of perfora- ted and two of imperforated platesfcwhile in Echi- nocrinus (Plate XVII, fig. D) there are four rows of imperforated plates, and the animal is really a crinoid, and not a sea-urchin. This (Fig. D) has a stem: that (Plate XIII, fig. C) has not. The Cri- noids are found in ancient geological strata—in the middle geological ages are those of (Plate VII). Free Star-fishes begin later in the geological for- mations. The Comatula or free Crinoids are again later (Plate I, fig A). The Echini appear long af- ter the families of Crinoids and free star-fishes have been introduced upon our globe. We have not yet one of the spherical Echinoderms before the deposition of the red stone or the Marchalkalk of Germany. And those spherical Echini or Cida- ris are the earliest ones, (Plate VI, figs. D and E.) Next we have such as have a central mouth, and in which the alimentary canal ends laterally. And at a later epoch those which have an elong- ated body (Plate VI. fig. B.) The first epoch in which elongated Echini appear is in the chalk de- posit. When there was not yet one free starfish, there were only Crinoids on earth. And what sort of Crinoids had we ? Not such as already resembled common starfishes (Plate VII.), but which resem- bled the lowest stage of growth of these animals, when they are still without arms (Plate XI. fig. £.) with irregular arrangement of their plates (Plate XVII. fig. C.) Next we have such which assume the shape of the star-fish, (Plate XVII. fig. B.) but are stilll Crinoids resting on stems with few irregu- lar plates, bat in which holes are arranged in a re- gular star above. And next we hare Echinocrtnus. 26 PROF. AGASSIZ'S (Plate XVII. fig. D.) that is, acrinoidal echinoderm aping the sea-urchins by its spherical form and by the regular arrangement of its plates and by the fact that there are zones of holes,alternating with zones of plates without holes. But that they are not echini is shown by the fact,that they rest on a stem, and that in each row of imperforated plates there are four sets of plates instead of two, as in Echini. Here crinoids are perfectly developed into the form of higher types, but under the general char- acter of the lowest group of these animals ; those forms, however, become more and more individu- alized in later periods. And here are other Cri- noids, (Plate XV & VII) from which free star fishes branch off during the subsequent geological times. But what is most curious, is the fact, that among the Echini, the oldest are the Cidaris (Plate VI, D) spherical bodies somewhat flattened, with large plates, narrow rows of holes, and remarkably largo Bpines in proportion to their proper size, (Plate VII, E) but precisely as we have them in the youngest condition of the true Echinus. (Plate XI, G).— The Cidaris are numerous before any true Echinus occurs. Next, those are developed and become gradually more and more numerous, and they are LECTt The result thus far obtained in the lectures which I have delivered, can be expressed as fol- lows : There is a gradation of types in the class of Echinoderms, and indeed in every class of the animal kingdom, which, in its general outlines, can be satisfactorily ascertained by anatomical investigation ; but It is possible*to arrive at a more precise illustration of this gradation by em- bryological data. The gradation of structure in the animal kingdom does not only agree with the general outlines of the embryonic changes. The most special comparison of these metamorphoses with full grown animals of the same type, leads to the fullest agreement between both, and hence to the establishment of a more definite progressive series than can be obtained by the investigation of the internal structure. These phases of the in- dividual development are the new foundations upon which I intend to rebuild the system of zool- ogy. These metamorphoses correspond, indeed, in a double sense, to the natural series established in the animal kingdom; first, by the correspond- ence of the external forms, and secondly, by the successive changes of structure ; so that we are soon succeeded by others of a more oblong form and those greatly elongated Echinoderms which we call Holothuriae, occur only in the present pe- riod (Plates XIV and V.) So that by all the facts to which I have briefly alluded, I can come to the conclusion that the class of Echinoderms presents, notwithstanding the im- perfect condition of our information upon this point, the most perfect agreement between the va- rious embryonic forms observed and the different permanent forms of the animals of that class in* their full grown condition ; that these embryonic forms agree also with the different structures of the fossil types through all the geological ages; and that these again in their order of succession, agree with the different appearances of the full grown living animals, or more precisely with their grada- tion as derived from a knowledge of their internal structure. These various relations, so complicated, and nev- ertheless so permanent in every respect, show the same thought throughout the whole—that struc- ture, development and order of succession in time, are regulated by one and the same unique princi- ple. here guided by the double evidence upon which the progress in zoology has, up to this time, gen- erally been based. Their natural series again correspond with the order of succession of animals in former geologi- cal ages; so that it is equally true to say that the oldest animals of any class correspond to their lower types in the present day, as to institute a comparison with the embryonic changes, and to say that the most ancient animals correspond with the earlier stages of growth of the types which live in the present period. In whatever noint of view we consider the animal kingdom.we find its natural series agree with each other: its embryonic phases of growth correspond to its order of succession in time; and its structural gradation, both to the em- bryonic development and the geological succes- sion, corresponds to its structure; and if the inves- tigations had been sufficiently matured upon this point, I might add that all these series agree also in a general way with the geographical distribu- tion of animals upon the surface of our globe ; but this is a point upon which I am not yet prepared to give full and satisfactory evidence, and which RE IV. LECTURES ON EMBRYOLOGY. 27 So much for the views referring to embryology in its bearing upon zoological classification. There is, however, another field in which the animal kingdom has been represented as developed according to the gradation of its structure: I mean the order of succession of extinct species in geolo- gical times. It has been long and generally as- serted, especially by the physio-philosophers, that the lower animals were first introduced upon our globe, and formed alone the population of the earliest periods in past time; that Polypi existed before MoFlusks; these before Articulata, and that Vertebrata were the last to make their appear- ance. But the discoveries in fossil Ichthyology, which it has been my good fortune to describe in my researches upon fossil fishes, have shown that vertebrated animals, fishes, have existed in the oldest epochs, and that such an order of succes- sion, as mentioned before, did not agree with the plan of creation. Indeed, that representatives of all the four great divisions of the animal kingdom, Articulata, Mollusca and Radiata, occur simulta- neously with fishes, in all the lowest geological formations, was soon ascertained by the investi- gations of paleontologists, and the fact of any reg- ular succession was afterwards altogether denied. However, the simultaneous occurrence of the four great types does not yet indicate the want of reg- ularity in the development of the various classes of the animal kingdom, taken isolately. Several eminent paleontologists, Leopold Von Buch, Count Von Murster, Sir R. Murchison, d'Orbigny, Prof. James Hall, and many others, have shown that the types of different classes which characterize the different geological ages, follow each other in an order which agrees with their zoological gradation as ascertained by structural evidence. The great difference between this fact and the views enter- tained before, consists in the knowledge of the in- k dependent gradation of the different classes, which : in the lower types arise all simultaneously, to un- dergo their metamorphoses simultaneously,through e all geological periods, whilst among Vertebrates, j. the Fishes were found to occur earlier than Rep- e tiles, and these earlier than Birds and Mammalia, jr which made their appearance last. It was in that a way shown that there is a progressive succession ' of classes among Vertebrata, ending with the cre- j ation of Man; whilst Polypi and Echinoderms among Radiata; Acephala, Gasteropoda and Ceph- r alopoda among Mollusks; Worms, Insects and , Crustacea among Articulata, existed simultane- ously during all great periods, and presented each a development of its own. However, another step had to be made to show a real agreement between the earlier types of an- imals and the gradual development of the animal kingdom, which has been the last progress in our science of fossils: namely, to show that these ear- lier types are embryonic in their character—that is to say, . that they are not only lower in their structure when compared with the animals now living upon the surface of our globe, but that they actually correspond to the changes which embryos of the same classes undergo during their growth. This- was first discovered among fishes, which I have shown to present, in their earlier types, char- acters which agree in many respects with the changes which young fishes undergo within the egg. Without entering i nto all the details of these researches, I will concluc by saying, it can now be generally maintained tha earlier animals corres- pond not only to lower vpes of their respective classes, but that their chief peculiarities have ref- erence to the modifications which are successively introduced during the embryonic life of their cor- responding representatives in the present creation, To carry out these results in detail must now be, for years to came, the task of paleontological in- vestigations. But the other connections mentioned above, I consider as established, and I claim these views as the results of my own investigation, though much has already been said upon the natural and suc- cessive development of the animal kingdom, and upon the propriety of introducing a classification based upon embryology. The views to which I allude are indeed not the same as those which I advocate; and in order to avoid mistakes in this respect, I will now dwell for a moment upon this point, with the hope, perhaps, to show that these views are incorrect, and must be given up, though they pretend to lead to a natural arrangement of the animal kingdom. The first notion of progres- sive development of the animal kingdom, of an agreement between the order of succession of types and their structural gradation, has been brought forward by that school of philosophers who in Germany take the name of nature-philosophers, (physio-philosophers.) But with them the idea of a gradual development of the animal kingdom, was by no means the result of investigations—was not the expression of facts, but was an a priori conception, in which they made their view of the animal kingdom the foundation for a particular classification, seeming also to agree with the little that was known of geological succession of types. Dr. Martin Barry, a distinguished physiologist in London, has however proposed principles for classification of the animal kingdom, which de- serve more particular notice, as he presents them as the results of his extensive investigations in embryology, and he has put his view upon the subject in the following words. Dr. Barry is one of the ablest investigators in this department, one of those who have most extensively studied the egg and its developments in the mammalia. To him and to Dr. Bischoff we are indebted for the most elaborate investigations upon this subject; but I am not aware that Dr. Barry has traced the metamorphoses of animals in other classes. His views are substantially expressed in the following statements: " There is no appreciable difference in the germs of all animals. There is a fundamen- 28 PROF. AGASSIZ'S tal unity in all of them." This isa result which is beyond all doubt, which is beyond all contro- versy. The eggs in the whole animal kingdom are identical in structure. However, this funda- mental unity must be restricted in one sense.— They are identical in structure for our senses, but we cannot consider them as identical in a higher point of view, as from each kind of egg there will never arise but one kin of animal; there is an essential, though not a r aerial difference in the egg from the beginning but in their material structure the eggs of ah animals are identical.— The first position must therefore be granted; but with the restriction upon which I insist, that though identical in structure, there is something which presides over the individual growth, from the be- ginning even of the formation of the egg, and makes each one give rise only to one sort of ani- mals. It could, then, just as well be said, that the eggs, though apparently uryform, are essentially different in different species. But Dr. Barry states that the class, or the characters of the class, be- come manifest in the egg in the germ, before the order can be distinguished. That is to say, that the first change which takes place in the embryo, is to bring forth in the new animal what charac- terises it as belonging to one particular class.— For instance, that a young rabbit would first as- sume the peculiarities by which it is referred to the class of Mammalia. Next, the order becomes manifest; but the family is not yet shown. The young rabbit would be distinguished as belonging to the gnawing animals. Next the family (here the family of Hares) becomes manifest; but,the genus not yet known. Next the genus (Lepus) obvious; but not the species. Next the species (Rabbit) distinct; but the variety nnpronounced.— Next the variety (white, grey, black rabbit) ob- vious; but the sexual differencesjscarcely apparent. Next the sexual character obvious; but the indi- vidual character not noticed. Next the individual character developed in its most special form. This is very logical, but not in accordance with nature; we may frame such a system in our closets, but it does not answer our observations. Let us remember what we saw in the egg, with which I began illustrating the growth of frogs.— Was it the character by which the frog is found to belong to the class of reptiles, which was first apparent ? By no means. '.It appeared first, under the form and with the structure of a fish, and not under the form and with the characters of a reptile. The lowest form of vertebrated animals was first developed in the earlier changes of the egg, before the class to which that animal belonged could be recognized. Not only would the first form under which the young Batrachian appears, exclude the class to which it will belong afterwards, but even the internal structure of the tadpole differs from that of the reptiles. They have no lnngs, no inter- nal serial respiratory organ, nor even a rudiment of it, and also no nostrils communicating from outside with this innner hollow sac. What did we find among the starfishes ? among the echini t— Did we recognize there the hard plates or the rows of regular plates which mark that class, of the rows of rfuckers ? By no means. Forms which would lead us to mistake them for Polypi or Medusa were first noticed, and not the indica- tions of their class; thus showing that there is no such thing as an earlier development of those characters which indicate the respective class of the animals under observation in the progress of embryonic growth. Next, it is said that the orders are manifest, but not the genus. But let us take as a test the em- bryo of a very well known animal among* mam- malia. To what order does the cat belong 1 To the Carnivora and to the family of Digitigrades.— What are now the characters of carnivora ? Sharp- pointed, canine teeth, with chisel-like incisors and various molars, the principal one of which is a sharp-cutting tooth. The claws again, are strong, curved nails, adapted for their peculiar mode of seizing their prey. Now, the young cat is already far advanced in its development before it has any teeth at all, and its paw is a real fin, with undivided fingers, and without nails in the earlier stage of growth. We have at first, therefore, not one of those characters which distinguish the order of Carnivora -and the family of Digitigrades; and nevertheless such an imaginary order of succes- sion in the development of parts is made the fun- damental principle of a system which is given as natural, though the whole is merely a logical par- tition of principles. The genus next should be shown. What are the characteristics of the genus, cats 1 To have four molars in the upper jaw, and three in the lower. But before the cat has all its teeth, the genus can be recognized, by its protractile and retractile clawB. The species indeed, is ascertained, is well characterized,by its peculiar form,before we can re- fer it to the genus, according to its zoological char- acteristic. But it is said that the variety becomes next obvious. The cat, however, may have already assumed a peculiar variety of color ; it may be a grey or a white, it may be of any color before the teeth, the characteristic of the genus, are fully de- veloped. And as for its individual character, the young kitten is playful, and shows its character long before its peculiar genus is marked out; and in short, every thing takes place in the reverse or- der from what it is supposed in this system. Nev- ertheless, such views are considered as suited to express the real gradation in the animal kingdom, from the simple reason that the whole statement seems natural and logical. A renewed examination of the metamorphoses of the frog will lead to the same conclusions, j^ first we do not observe changes indicating the class to which that animal belongs, but such char- acters as would rather indicate the class of fishes • nor are the characters of the order of batrachian* LECTURES ON developed before the young animal assumes forms related to genera to which it can never be refer- red. Indeed, the tadpole has all the peculiar ap- pearances of batrachians with permanent gills, be- fore a frog can be recognized ; it resembles suc- cessively Menobranchus, Triton, and Menopoma, before it loses its tail; and as for toads, they have webbed feet, that is to say, they resemble another genus, the frogs, before their fingers are entirely separated, though the species can be recognized in the distribution of colors long before. (Plate III—Frogs) Professor Milne-Edwards, of the Jardin des Plantes.has proposed similar views, and indeed ex- pressed in nearly the same words, his conviction about the gradation of the animal kingdom; but not with reference to the development of zoologi- cal characters, but with reference to the changes which the animals undergo in their structure. He 4 EMBRYOLOGY. 29 has referred his views more particularly to the structure and the development of the functions of animal life; and from this circumstance his views agree better to nature, when he says that those organs are first developed which are more impor- tant to life. However, strictly speaking, it is not absolutely true. It is the nervous system which we may consider as the organ most important to life; and it is not the nervous system which becomes first apparent in the embryonic changes. The sys- tems by which the body grows are developed be- fore those by which it lives a higher life come into play; so that, though in a general way, the organs i most important to existence are really developed | first, it cannot strictly be said that they are the I higher organs which are developed first} and that I the special differences which characterise families | and genera should be engrafted as it were upon a fundamental plan. | My aim is an entirely different one, as you may have perceived from my first lecture. It is to show that in the real changes which animals undergo during their embryonic growth, in those external transformations as well as in those structural mod- ifications within the body, we have a natural scale to measure the degree or the gradation of those full grown animals which correspond in their ex- ternal form and in their structure, to those various degrees in the metamorphoses, and therefore to make the metamorphoses of animals, as illustrated by embryonic changes, a real foundation for zoolo- gical classification. Let me only mention that on the whole, the high- er families of the various classes of the animal king- dom are distributed over the warmer parts of the present surface of our globe, and that the lower families are rather numerous in the milder and colder regions. Thus among mammalia,the Mon- keys are strictly circumscribed within the limits of the growth of Palm trees; the large carnivorous beasts prevail in the tropical regions; whilst the sheep, goats, and oxen are natives of the temper- ate zone; among reptiles,the crocodiles occur only in the warmest countries; whilst the lower Batra- chians, those with external gills or permanent tail, extend even far north. There are, nevertheless. infsrior families which are also strictly tropical; such for instance as the Pachyderms, and to some extent, the Edentata; but this fact has doubtless reference to the early introduction of these fami- lies in the plan of the creation, during a period when the surface of our globe was warmer than it is in our days; so that the location of their modern representatives in the torrid zone, can be consid- ered as merely determined by the peculiar adapta- tion of their general plan of structure for warmer climates, rather than related to the gradation of the types, according to the present condition of the distribution of heat upon our globe. The induce- ment for their present location is not their higher structure, but their relation to earlier types. But now, I proceed to illustrate the history of 30 PROF. AGASSIZ S another class, that of Medusae; the next among ra- diata, whose embryology we have to investigate. But it is out of the question to understand the changes which Medusae undergo,without knowing their structure, and this structure is not only very complicated, but it has been little studied and is still obscure. I stand, therefore, with a very diffl cult task before me, and I ask your indulgence upon this point. Let me begin by pointing out a few diagrams, and saying a few worfis upon the figures before you. (Plate XIX—Young Medusa.) Here (Plate XIX, fig's A B G) are outlines of a family which has been described by Sars, the dis tinguished Norwegian naturalist, as a peculiar polypus, under the name of Scyphistoma. Here (Fig. I) are other figures, which have been also described by Sars as polypes, under the name of Strobila. Here (Fig. J) is another free animal, de scribed under the name of Ephyra. And here (Fig. M) is another, found on the shores of the At- lantic, both in Europe and in the United States, in the temperate zone, which belongs to the genus medusae. As to the class to which these various animals belong, I may mention that the two ge- nera, Strobila and Scyphistoma. were referred to polypi, and the other two (Fig. J M) to jelly fish- es, or Medusas. Now, gentlemen, it has been as- certained within a few years, both by Sars and Yon Siebold, that all these figures are the various stages of growth of one and the same animal.-— We have here (Plate XIX) the metamorphoses of one and the same animal—changes which take place in the growth of an egg. This (A) is an egg,as it is laid by a Medusa. Here (Plate XX, A? (Flate XX.—Polypi—CoRTNiE, Syncortn-b, Podocoktnje ) we have a still more extraordinary structure (syncoryna). You see these stems terminated by a rosy colored head, from which tentacles, half a dozen or more, arise, and out of these various bo- dies, a little tubercle here, a more prominent one there, and another bell-shaped here, with tentacles around its opening. Here is another form (Fig. B) called Podocoryna, by Sars, from which va- rious kinds of buds arise, which do not resemble the primitive stem; also much larger buds, which differ still more, and which are at a certain time freed and grow into other animals. Indeed, stems of polypi, from which arise buds of medusas or jelly-fishes, budding from polypi-like stems, be- coming free and growing into a regular, simple, isolated jelly-fish, like this (Plate XIX, N); this is the case here, (Plate XX) a bud which grows into a jelly-fish. It is, however, out of the question, that in its different stages of growth, an animal could belong to various classes, or that an animal of one class could give rise by budding to animals of another class. Therefore, it is perfectly obvious, from the nature of these well authenticated facts, that there has been a want of understanding of these phenomena when they were first described; and it was not until a few years ago, when Steer- strupp found out the key to this astonishing com- plication, by ascertaining that there is an alter- nation in the mode of reproduction of many ani- mals, which takes place in different ways in the animal kingdom. In some, there are eggs laid, which eggs give rise to animals different from their parents, and these in their turn give rise to eggs, from which arise animals similar to their grandparents and different from their parents.— In other cases, animals lay eggs which go to form individuals different from themselves, and these individuals, by budding,or transverse division, pro- duce forms which are freed, grow, and then re- Bemble the parent, by a complicated process of metamorphoses. However, though Steerstrupp, for the first time brought out these conclusions distinctly, he was' LECTURES ON EMBRYOLOGY. 31 stvnc»bat anticipated by Sars, by Sir John Daly- ell, and by a French naturalist, Du Jardin; though they did not carry out their investigations to the same purpose, yet they led the way in the same track. How these changes take place, will be I suppose better understood if I begin by giving an outline of the structure of these animals, which it has been possible for me to examine more com- pletely than it had been doire before; availing my- self of several small species which live in Boston harbor. The large animals are not those which are best suited to such investigations; when large their bulk prevents their being examined under the microscope. But let the animal be small enough to be placed entire under the microscope and you get a general view of the structure ; and by apply- ing a higher power to the various parts, you can trace the details in such a way as to ascertain most completely their organization. Such was the process by which I was enabled to discover in these minute medusae, even the nervous system, which had been only suspected, but not traced in its distribution. And let me add, that beside their physiological interest, these animals are wonder- ful in their aspect, and present the most attractive wght which can be witnessed, Their transparent, delicate bodies swimming freely in the water and moving regularly by the contraction of their whole mass—the elegance of their outline and the diversity of the appendages which hang down from their globular body—or the suckers which rise from the centre, and constitute other appendages from the middle of the sphere—all these contribute to make these animals wonderfully beautiful. An in- creased interest Is felt when seeing at first scarcely an outline, so transparent are they, and discovering afterwards by the simplest process of examination, ■consisting in modifying thelight which passes from the mirror of the microscope through their body, all the diflerenees of structure to easily overiook- «d at first sight. And again, they belong to a class of which so many are transparent, or phosphorescent,that there are endless inducements to investigate these ani- mals. Here are various figures (Plates XXI, 'II, FII, 'IV, 'V, 'VI, 'VII), all representing Medusa. Many of these figures are of a hemispherical form, as plate XXI; and this form (Plate XXVII, fig. B) In the margin of this form (Plate XXI, fig- A) you see we have two kinds of appen- [Plate XXI—Medusae.] [Plate XXII—Medusa. [Platk XXIIl—Mbhum. dages, aud you see (tig. ii) tUat there is a central cavity, and that there are four bunches of a pecu- liar character here, the ovaries, (fig. C), and that the lower surface presents various rays diverging towards the edge. In another form, Beroe, (Plate XXII) we have a tubular body with vertical rows of vibrating cilia, and a wide opening below the internal cavity, which is more complicated than that of the other types. Here is (Plate XXVI) an- other, Agalmopsis, which is still more complicated, from the diversity of all the appendages which hang from the main stock; and here is another, which 4s, if possible, still more complicated*, and has a very large vesicle above and numerous ten- tacles hanging below. This animal (Plate XXIII) is known to the sailors by the name of Portugese Man-of-war. Naturalists call it Physalia. Others are flat, circular, or oval, with several rows of sim- ple appendages, as Velella, plate XXIV, and Por- pita, plate XXV. [Plate XXIV—Mr- toa.] f.-utcsaOf i^aciiscliull, WUO llab olUUiCU lucoe animals more extensively than any one else, has divided them Into three groups—Ctenophora, Dis cophora, and Physophora. Those which have these vesicles, by which they are suspended in the water, are called Physophora. They are all con- sidered as simple animals, though their form is extremely complicated Here is an enlarged ig- 32 PROF. AGASS1Z S [Plate XXV—Medusa ] are (Plate XXVI) of one of these animals, with all various appendages—tentacles, suckers, groups of eggs, and all sorts of vesicles—forming one elongated body, with fringes. It is the Agalmopsis of Sars. Let us now see what is the structure of these animals. The internal cavity communicates with the exterior by a broad open mouth, as you see in this sketch, or by bunches of tentacles which terminate in little suckers, as you observe in this figure (Plate XXVII, fig. B)—numerous suckers hanging down from the central appendages and forming as many mouths, as many openings com- municating with the central cavity, as there are such appendages. In another case (Plate XXVII fig. A) we have simple little openings, or pores, up- on the surface of the larger appendages, all direct- ed inwards, uniting and combining to form larger stems—finally combining into fewer tubes and emptying into a main cavity, and from that main eavity branching off again into numerous tubes, and dividing over the margin of the disc. Those ramifications from the central eavity towards the surface can be easily seen by holding the light in a certain angle before these animals in their living condition. And by injecting colored water, you may nil them in all directions, and see that there is, as in plate XXVII, fig. A-, a net work of vessels ramified around the animal. There are others. [Plate XIX., fig M.] in whieh there are main stems, which divide into some few more towards the margin, or nnite again into a circular canal all around the edge. There are even some in which the central cavity [Plate XXVIII.J is very small, having only a little sack at the summit of a long proboscis.which is the mouth ; the little sack next divides into fonr tubes, which then extend towards the edge, where they unite again to form a circular tube. Liquids are constantly circulated in these cavities The food is digested within that cavity and then circulated through the tubes, and in those which have only minute pores as oral apertures, the food can con sist only of microscopic animals, or of decomposed organic matters—in others which have a larger mouth, larger animals are introduced. In the small species of Boston harbor, [Plate XXVII. flg. C.J which was first described by Dr. Gould in his Report upon Invertebrate Animals of Massachu setts, and which will be exceedingly common in a a few weeks, I have seen this proboscis hanging down and stretched three times the length wnich you see here; and after it had swallowed some- thing, and the food had been digested, the globules arising from the digestion would be circulated through the tubes and would be seen under the ml croscope most plainly, diverging towards the mar- gin of the sphere, there moving into the circular tubes, or perhaps even moving down into the ap- pendages—those hanging arms which are hollow— and again trace back their course into the circular tubes; some of the globules would disappear when absorbed by the surface, but the remainder is cir* culated forwards and backwards—to and fro—in those tubes before disappearing entirely. Such a structure cen be considered the lowest condition of a system of circulation, which is at the same time a modification of the alimentary tube, where the stomach divides, and where the divided stomach again unites into vessels—into common vessels, which branch in their torn. Here we have the tubes uniting and then branching off again, [Plate XXVII. fig A.] but in Fig. C there is a distinct mouth and proboscis. The mass which forms the body^in medusa is transparent and cellular. And then there are distinct muscular fibres of two kinds, circular ones around the whole disc, and radiating ones, which form distinct bundles diverging from the centre towards the periphery; in those medu- sa which have four diverging alimentary tubes- the main radiating muscular bundles alternate with the tubes (Plate XXVII, fig. C.) All these muscular bundles and the circular fibres contract alternate- ly, so that the body can be shortened or flattened in various ways, and thus, through the agency of these muscles, the animal moves in all directions upwards, sideways and downwards, at will. That these anfmals moved by contraction, had long been observed; but the existence of regularly arranged muscular fibres in the class of medusae, was still doubtful. When Ehrenberg published his investi- gations upon the structure of the medusae of the Northern Seas, though he concluded that there must be muscular fibres, he could not discover a regular, complete, muscular system. However in these small mednsse, the muscular fibres av« laraa LECTURES ON EMBRYOLOGY. S3 f Plate XXVII-Medcs*;. enough to be seen in the living animal, under a power of a few hundred diameters. Beside this, there is around the upper part of the alimentary tube, a linear circle of another sub- stance, from'which radiate four threads, following the direction of the alimentary tubes, and ex- tending towards the periphery, which reach there the spherical, colored bodies, now generally con- sidered as eye specks, and uniting with each other, form a circular thread all around the margin of the disc. This apparatus I consider to be the nervous system. Its position is the same as in the other radiated animals, a circle around the alimen- tary tube, with diverging rays, ending in the small colored organs which since discovery (Plate XIX, fig. M) have been considered as Ehrenberg's eye specks, similar to those which I have already no- ticed, at the end of the rays of star-fishes, and upon the plates of Echinoderms. The fact of these threads going to those spots (Plate XXVII, fig. C), leaves no doubt, in my mind, that it is a complete radiating, nervous system, similar to that of star- fisnes. So that the structure of medusas, though peculiar in itself, by the remarkable mode of dis- tribution of its inner cavity, which does not con- stitute an alimentary canal proper, resembles al- most entirely the structure of Echinoderms, and eonstitutes one of the main classes among Radiata as Echinoderms do< The position of these ani- mals was mentioned. They swim free, the mouth downwards, the sphere upwards; and this is al ways the position which the Echinoderms assume. The Echini, Sea Urchins, walk about, the mouth downwards. Starfishes walk about, the mouth downwards. The Crinoids, however, stand up- right, the mouth upwards, and this is the position which the animals of the lowest class assume. In all Polypi, the main body stands upon a stem, the mouth upwards; and we have also among Me- dusae [Plate XIX, figs. G & I] a similar condition during one period of growth. When the young animals are fixed by the lower portion of their body, the tentacles, or appendages, which every where hang downwards, stand here upwards; so that you see how remarkably the lower types among Echinoderms resemble in this respect the Polypi in their constant position, and how in youth, Medusae, in that respect also agree with Polypi. There is a constant recurrence of charac ters from one of these classes to another. They are interwoven in a most remarkable manner, All Jelly-fishes are generally cons;dered as simple animals; but I am satisfied that there are, on the contrary, highly complicated ones among them. The Physophora differ indeed widely from the other Medusa, by their diversified appendages, as is shown by the structures figured on this diagram. [Plates XXIII and XXVI ] I am prepared to show that these are compo'und animals, composed of groups of individuals of different kinds ; indeed, compound animals as we find them among Polypi. [Plate XXVIII—Hydra-Campanularia ] lu order to show that this is the case, let me il lustrate in detail the metamorphoses of Medusae. Let me also refer you to some Polypi, [Plate XXVIII] in which you see how individuals are combined together, forming a compound stick. Though all these individuals are of different ages and have been found successively, they form living colonies, as it were, of successive generations, uni- ted by material connections, which remain for life —the new individuals not separating during life. In others, the successive buds may be more or less different, and nevertheless remain united in one common colony, or as it were form a community of individuals closely united, though differing in age, size, form, and even In sexes. Such is the case at least in the Campanularia, figured Plate XXVIII. But there are also among Polypi simple ones, like this little Hydra, 1 Plate XXIX]. When alive the Medusa lays eggs, and the em bryos are hatched, these germs swim freely, and then become attached. And the point by which they become attached grows longer [PI. XIX, B], in proportion as the mass above grows larger The- 34 PROF. AGASSIZ S [Plate XXIX—A Fresh Water Poltpus, with a simple Cavity and a moveable Stem.] vibratory cicil a, by which they first moved, are finally cast. There is a depression forming upon the summits, and then two little horn-like appen- dages grow out. [Fig C] They grow larger. [Fig. D.] The tentacles grow longer, the depressions still deeper, and then there is finally a central cav- ity with four distinct tentacles. [Fig. E.] Then there will be a little Hydra like animal, with eight tentacles, a cential cavity, and a peduncle by which it is attached. [Fig. F ] This is the first developmentof the germ of the common medusae, the jelly fishes of this shore, which are known in Boston harbor under the name of sun-fishes. When it is grown somewhat larger, a contraction takes place under the rows of those tentacles, which have become more numer- ous. In this stage of growth buds may also be found. (Fig. H^ New individuals may thus arise from buds on the sides of this simple stem, and these new individuals may grow to a consid- siderable size with the parent stalk before they separate. But at last they will separate, and grow by themselves and form new sticks. So that we have here two modes of reproduction among me- dusae; in the first place, from eggs, which grow into polyp-like animals, (PlateXIX, fig. A—F) and secondly, by buds which will produce new individ- uals, (fig. H.). The bud, being-separated from the main body, will even form new colonies, and so on, (Fig. H.) At first these buds differ somewhat from the parent stock, but soon assume, the same character, differing slightly when they are finally freed. There are animals in which the successive buds differ much more. There are in this (Plate XXVIII) Campanularia, as it is called, buds which give rise to animals with large tentacles, and there are oth- ers with shorter tentacles, and there are even othersofadifferenttype; so that the various buds which grow from one stock may differ widely and yet be buds of one and the same stock.— Herein the young Medusae (Plate XIX) we see that only one kind of buds arise—but there has been still another mode of reproduction and multi- plication observed in the same animal (Plate XIX, fig. I). The stem, on growing longer and higher, (Fig. G ) will begin to divide by transverse contrac- tions into articulations. There are at first, simple folds noticed in the skin, scarcely deepened to any extent, but gradually growing deeper and deeper, so that at last it seems as if a pile of discs were heaped upon each other, (Plate XIX, fig. I,) the lower part of which is a simple stem, as in Fig. G, and the upper part, still consisting of a row of ap- pendages as they have grown upon the summit of this little Polyp and Serrate (Figs H and G). Next, the edges of the discs begin to be fringed, (Fig. I,) the cut growing deeper and deeper, thf se serra- tures assume a regular form, and the contraction growing successively deeper and deeper, those ser- rated discs, almost separated from each other,form a pile of loose discs simply connected by a central axis. And as soon as the Polyp has divided into this series of discs, the upper tentacles, that is to say, the tentacles of the primivite Polyp, with the upper disc, die away. What formed first the prin- cipal pare of the growing animal, dies away, ex- cept the basal attachment,which remains; and next, in the remaining pile, the uppermost disc frees it- self from the pile and begins to swim. But the moment it is free it assumes an inverted position, (Fig. K) ; those fringes which were upwards, now are turned downwards. The inner surface, which was first upward, is now downward also. In this way, a series of these serrated discs (Fig. L) are successively freed from a primitively undivided stem, by gradual transverse articulations, to form as many independent individuals (Fig. T), which after all can be traced to one single egg. There are finally quite a number of individuals formed, which have arisen simply by transverse division, and by the successive modifications which each of these discs has undergone. And after freeing themselves, the Ephyrae, as they are called, (Fig. J M) will undergo such changes as to assume those structural peculiarities which char- acterise the perfect Medusae. The tube will be* come hollow. The cavity will enlarge, and that will have its tubes, branching into the disc by va- rious canals, (Fig. M ) Those canals" will circulate fluid around the disc, and finally the complicated structure of Medusae (Plate XIX. Fig. M.) i8 pro. duced by the addition of fringes on the edge; and the growth of processes on the side of the stomach which give rise to the egg, the eggs always hamr. LECTURES ON EMBRYOLOGY, 35 ing from the sides of the stomach, being, indeed, simple pouches from the stomach. I ought to have mentioned before, that the eggs in Medusae are universally formed in connection with the ali- mentary tube, and that in some of them, as the small species of Boston harbor above described, they are simply diverticula of the digestive cavity, forped in ccecal appendages of the same, to be- come free, independent eggs afterwards. Their position varies even most remarkably along the alimentary tubes, in spme, that before mentioned, being developed along the central proboscis ; in others, the Stomobrachium, being formed in four bunches along the four tubes diverging from the central cavity. Their mode of formation in such positions has nothing more to astonish us, since we know, from the investigations of Sars, that there are Medusae, the Cytbers, in which new indi viduals are developed from buds arising from the stomach. At a certain epoch the whole genera- -tion produced, arises by transverse division of the stem derived from the eggs of the Medusae, pro- ducing a number of connected individuals, from the sides of the primitive stem (Plate XIX. Fig. H); there are also often found buds growing upon the lower portion, but invariably, at some period, the perfect Mudusae will produce eggs. In some Polypi we have also eggs arising from the sides, like buds as in Hydra. [PI. XXIX.J We have here. [Plate XX] from Polyp, Syncoryna and Podocoryna buds arising which differ entirely from the main stock, but which are successively freed from it, and which give rise to animals which are metamorphosed into real Medusae. Instead of be- ing considered as Polypi, those beings should no longer be considered as perfect animals—should no longer be arranged in our systems by them- selves, any more than Ephyra, the larva of Medu- sae [Plate XIX fig. J.J; any more than Strobila [Fig. I.J or Scyphistoma [Fig. E.]. They are only to be considered as the stages of growth of Medusae; in some of which the regular Polyp divides into many buds, forming as many Medusae [Plate XX fig. B ], or in others, of which simple Polypi give also rise by budding to regular Medusae, there being simultaneously other modifications of the process- es of budding introduced, by which the animal is finally brought to its higher metamorphosis, [Fig. A.J; the budding being [Plate XX fig. B.] the step by which the higher metamorphosis is introduced. The free individuals, which differ so much from the parent stock, being finally cast off. In Medusae proper the budding does not intro- duce the higher metamorphosis; this taking place only in the individuals formed by transverse divi- sion. Now, let us for a moment compare such a being as Agalmopsis (Plate XXVI) with the dividing stock of Strobila (Plate XIX, fig. I). We see at once that their position is inverted. Here (Plate XXVI) the fringes hang downwards, but here (Plate XIX, fig. I) they are upright. To institute a close comparison, we must therefore consider them in the same position, and the resemblance will be striking, especially towards the narrow end. But when we know that in Polypi buds of various aspects can arise from one stem, and remain con- nected with the cavity of the main stem, as it is here shown inCampanularia (Plate XXVIII)—the connecting axis being the main body with a con- tinuous cavity which extends into the branches —we have no reason to wonder at a similar growth in animals like Strobila (Plate XIX,figs. G and H) where there is also a similar connection between the bud and the main cavity of the body. And now in Agalmopsis (Plate XXVI) instead of considering those various appendages as organs of a simple animal, ldt us for a moment inquire if we could not consider them as buds of various kinds remaining around one stock, and forming a community of heterogeneous individuals, living a common life, in the same manner as in polypi, where we have observed individuals, though some- what heterogeneous, living also a common life. And if this comparison can be carried out, we have established that Agalmopsis must be consid- ered as a community of distinct individuals. Now, what are, in the first place, those largest bottle-shaped appendages ? They are considered as suckers. But they are suckers whiph pump food, which digest it in each of these bottles.— There is a cavity in which the food is digested; and the result of this digestion is circulated through the main tube. It is a condition identical with the condition of the polypi, in which a new bud arises to remain connected with the main bo- dy, to have, however, a cavity of its own in which to digest food, and then circulate it with the main mass. Here (Plate XXVI) is another kind of suckers, but performing the same function. They are similar individuals in a lower degree of growth. At first these bottle-shaped open suckers are small, simple appendages from the main tube.whicb grow larger and'finally assume a more individualist life, so that we would have eating individuals upon a common stem, which provide the whole communi- ty with food. They are the mouths, the eating in- dividuals— other appendages which seize upon the prey and which bring it to the suckers, may be considered as compound stems. Of these apporates here is one highly magnified: you have first, the bottle-shaped apporates with their various modifi- cations. Here we have the nettling organs, which are,when highly magnified, also bottle-shaped, and from which threads hang down. They are another kind of individuals, suspended by their peduncles and from which fringes hang down—but not sim- ple individuals. They are individuals which bud in their turn, so as to form groups of individuals- groups of catching individuals. Then there are other buds, which remain hollow cavities, and are considered as vesicles to suspend the animals. It is the swimming apparatus of the 36 PROF. AGASSIZ S body; but this form resembles so much that of the suckers, that they must be considered simply as a modification of them And if the suckers are buds, these must be closed buds. Then there are still other buds, which re- main closed, and which gradually swell and sink. They do not assume so much individuality as to open outside, and to peform other functions. Ad- mitting simply the fluid within, and pushing it out again into the common cavity. Such buds are imperfectly developed individ- uals, performing the function of respiration. They are individuals to breathe, as there are in- dividuals to seize the prey; as there are individuals to digest, living upon one common stock. There are other individuals which bud also, and they are ovaries. Here, Fig. XXVI, apparently an organ, but nevertheless, arising like the other buds—re- productive individuals, and of these there are even two kinds—such as assume the form of hunches of grapes, and others which assume the form of those small Medusae here, (of Plate XX. Fig. B) and which occur, especially in the lower portion of the animal—swim away freely, and re- produce free individuals. Now if it was not for these cases—such buds which may reproduce the whole colony—such a conclusion as I am about to present would seem untrue. But, when there are some among these various buds which actually present the structure of medusae, we must conclude that the so-called After illustrating the structure and embryonic development of the Jelly-fishes, I did not draw any conclusion in my last lecture as to the natural classification of these animals; because I wanted first to examine more closely the class of Polypi, in order to trace, if possible, defined limits between these two classes. Indeed, there is great difficulty in ascertaining the proper limits of the class of Poly- pi as a natural division of the animal kingdom, owing to their low position in the series. Their structure is so simple, that they are apparently re- lated to all the lower types of other divisions. And Indeed we find that animals bf very different types have been referred to the class of Polypi. There have been articulated animals brought in connec- Physophoridae are compound animals, in which the various functions of the body of medusae are distributed to different individuals in a most diver sificd manner, they being, however, not organs of one animal, but of a community of individuals.each performing special functions; the whole exempli- fying what a well regulated Society should be. There is the most remarkable resemblance be- tween the mode of association of individuals in the compound animals which throw out buds, connec- ted with the primitive stock, and the plants which produce successively buds of different kinds. In- deed the branching of trees from buds compares in all its features with the budding of compound ani- mals, and the similarity is closer in proportion as there are more buds of different kinds produced, which through life are confined to particular pur- poses ; for instance, plants which produce similar buds, growing into branches, identical with the main stalk, will compare with the simpler forms of compound animals, in which all the buds produce individuals similar to the primitive stem. Plants, on the contrary, which produce at various periods leafy buds, and flowering buds, in which the male and female flowers may even be separated, will compare more closely with compound animals, consisting of heterogenous buds which remain gen- erally united for life, and from which only from time to time eggs, or peculiar buds, are detached, like seeds, to produce new individuals and new communities. tion with them. There have been Mollusca re- ferred to that group. And even at the present moment, after anatomical investigations have thrown so much light upon this subject, I incline to admit that the class of Polypi, as it is now cir- cumscribed, is by no means a natural one • and in- tend this evening to show that entire groups, con- sidered by all naturalists at the present moment as Polypi, will have to be removed from that class and that other types, which are referred to other classes, will have to be combined with this class.— It will be perhaps best to begin this illustration by pointing out the various forms which are thus combined at the present moment as one class un- der the name of Polypi- LECTURE V. LECTURES ON EMBRYOLOGY. rP'ATE XXXrV-VERBTiLLTTM.J ] [Plate XXXVI-Retepore ] 37 ^^ VVe iuic ucic diagram- (uucul vvtileu, Vcielil- lum, is given in Plate XXXIV) of the principal groups of this class; and indeed there is scarcely one family of Polypes of which these diagrams do not represent some species. The Corals are among those which have from the beginning been consid- ered as a type belonging to the class of Polypi — And various species are represented here4 among them are stems, branching and supporting soft Ut- ile animals, which come out like flowers. The variety of these beings is such that indeed Chey rival, by their glorious colors and variety of Form, the most brilliant flowers of the dry land.— Such as this Actinia are common on these shores, and have also universally been considered as Poly- pi ever since these beings haye been combined into one class, and have been separated from the vegetable kingdom. It would carry me too far if I were to give now the full history of the knowledge successively ac :uired upen these animals, and to refer to those views of these beings which were entertained by naturalists at the time when some w^re supposed to be simple mineral concretions, and others were considered as marine fiowering plants; the ani- mals upon the stems being mistaken for flowers, and the stems compared to the stems of plants. Bet after it was ascertained that there were con- tractions taking place in the soft parts, that there was an internal cavity iEto which food was intro- ■ duced and digested, no doubt could remain as to the animal nature of these beings; and all small animals whose upper opening is surrounded by tentacles, and whicti are grouped together upon a common stem, were atonce referred to thatclass. And some simple animals, like the Actinia, were also referred to the same class, being considered as isolated forms of the same character. But we see upon the following Plate (Plate XXXVI) one of these coral like stems, (Retepora) with mi- nute openings, in which numerous animals are rontained, whose structure has been investigated by MM Audouin and Milne-Edwards, and has been found to differ so materially from that of Polypi, that this typo, of which there are various >"orms, is now generally considered as belonging C« the great division of Mollosca, although tibey are compound animals. All the investigations which have followed since this suggestion Was first made, have only gone to confirm the view, that these porous animals do not belong to the class of Polypi, but to a higher type, and indeed resemble in some respects even the oysters, the clam3, and still more the compound ascidiae, in whose vicinity they will in all probability be placed forever, showing that compound animals may be* long to all great pmups of the animal kingdom, and even occur as anomalies among mammalia, in the shape of twins. [Plate XXXI—Alctonium and Rfntlla ] Oilier diagrams represent various 01 tier types. Here, (PI 30) for instance, the beautiful Tubulariae are seen forming most beautiful flower-like animals uniting in bouquets upon the old logs and swim- ming lumber which are fastened in the water. Two species of this kind are very common around the city of Boston. One (Plate XXX, fig. G) with a larger crown, occurs in great abundance upon the logs in Craigie's bathing house; another smaller species is found almost everywhere upon old logs. The larger is about two or three inches high, and the crown, when fully expanded, about one inch in diameter. This diagram, (Plate XXXI, fig. A) represents another still undescribed species, with compound stems, from Boston harbor, belonging to the fam- ily of Alcyonium, in which every one of the indi- viduals terminates with a star of eight fringed ap- pendages or tentacles (fig. B). The most curious, however, is this one (fig. C), a Renilla, which f collected in Charleston, S. C—an animal with a soft body of a hollow stem, sticking in the wet sand, with a large disc, spreading above whichs seen from below, shows lateral dilations, from which, -upon the upper surface, arise a great etaer 38 PROF. AGASSIZ'S Plate XXX—Tubulartje.1 isolated lime rlowei-liKe Polypes vtig. C), ot which one is figured (fig. D) upon a larger scale, showing that the tentacles, eight in number, are also fringed like those Alcyonium, being regularly arranged in three pairs upon the two sides of the elongated mouth, a seventh and eighth tentacle being in the prolongation of the oral aperture. This animal is of a beautiful purplish color, emitting in the dark a most wonderful, soft, greenish-golden phosphor- escent light. There is another type of Polypi very common on the shores of Massachusetts and farther South, the Actinia, of which one species (Actinia Margi- nata, plate XX, D) is found upon logs along the wharves in Boston harbor and upon the rocks at Nahant, in great numbers. They are isolated ani- mals, growing to a comparatively larger size than the other Polypi; remarkable for their extraordi- nary contractility, the body assuming constantly new forms and new positions; now entirely drawn out in the shape of an elongated tube with a circle of tentacles around the free extremity, (Fig. D) then the tentacles rising and falling, or shutting in and expanding; next shortened and contracted with the tentacles closed (Fig. E); or the external envelope entirely shut over the inner part, resum- ing then a hemispheric shape, like round tubercles sticking to the ground by their fleshy base. The variations of color are as numerous as the changes of form; upon the same 6pot may be seen brown ones, and others dark brown or blackish, yellow- ish, purple, salmon, rose-colored and more or less mottled, the tentacles presenting alternations o! dark and lighter rings, or at least having thelv tips differently colored than the lower part. That Velella and Porpita, now generally ar- ranged among Jelly fishes.will have to be removed from the class of Acalephae and placed side by side with the Actinia, will not escape the attention of those wlro are familiar with these animals. Recently, the Polypi have again been extensive- ly investigated by Prof. Milne-Edwards, whose name is always to be mentioned when speaking of the lower animals, as scarcely any one has done more than he has in their investigation. Enron- burg has also largely contributed to our knowl- edge of the Polypi. Bu£ no one has done more to illustrate their natural history than Mr. James Dana, of New Haven, Conn., who accompanied the exploring expedition under Capt. Wilkes, and who has published the most elaborate work upon this • subject which has ever issued from the press. A work, indeed, which will remain a standard of au- thority in this department for many years to come- The embryonic growth of the^se animals has been studied almost exclusively by Naturalists living in countries which have been wanting in facilities for investigation, and are deprived of privileges which Naturalists have enjoyed in other parts of the world, where the animal kingdom is more luxuriantly developed. It is on the shores of Norway and Sweden that the most important investigations upon the em- bryonic growth of these animals have been made- There, where the observer is neither attracted by the variety of animal3, nor by the possibility of discovering easily new species, the interest of the subject has drawn them into a deeper and more thorough channel of investigation, which has en- dowed science with a more extensive acquaintance with all the difference of structure which is shown by the animals of those shores. And, indeed, far from considering it an advantage to be placed upon a shore where new treasures are thrown abundantly into the hands of investigators, I think it is, on the contrary, an unhappy inducement for observers to devote their whole attention to the multiplication of specific distinctions, without al- lowing time for the more important and more ex- tensive investigation of the physiological phenom- ena attending the life—attending the development of those beings. The structure of the Polypt can be best exem- plified in the Actinia (Plate XX, fig. !>) as they are among the largest, and as they are now more extensively illustrated than any other type of the class has been before. And what I have to say of these animals will be scarcely more than a repetition of what Dr. Jeffries Wyman has pub- lished in the work of Mr. Dana, already mention- ed ; some few observations only, the result of my own investigations, having been added to .his since the publication of that work. The body it • of rather large sire for a Polype, measuring fr0BS LECTURES ON EMBRYOLOGY. 39 'jPlate XX—Polypi Actiniae, Cortn^, Stn- CORYNiE, PODOCOBAN.*:.] one to several inches in length when fully ex- panded ; it consists of a membranous sac, as in all Polypi, with numerous tentacles round the upper extremity, and contains within, another sac, open- ing above between the several rows of tentacles. [Plate XXXII—Polypi—Actiniae.] Ia\ In this drawing (Piate XXXll, tig. H) you notice the whole structure in a vertical section of the an- imal, in which the relations between the different parts and their interior cavities are at once seen. You notice the external walls of the animal, and the rows of tentacles forming the upper outline. And from the centre, where there is a large open- ing which must be considered as the mouth, hangs down a thin sac, suspended within the cavity form- ed by the external arms and the surrounding thick envelop of the body. This sac is a stomach; it is maintained in Its po- sition by internal radiating membranes, extending all around the mouth and stomach and uniting with the external envelop so as to divide the interven- ing space into many chambers. There are also shorter folds which penetrate from the external walls towards the centre, so that the space between the stomach and the lateral walls is not one con- tinuous cavity, nor uniformly divided into equal chambers, but it is a cavity divided and subdivided into wider and narrower spaces by partitions which extend either entirely across the cavity surround- ing the stomach, or only partly into it, thus form- ing imperfect chambers; all the them, however, remaining connected by the open space which is left free of divisions below the stomach. Here is a diagram [Plate XXXII fig. A] in which the ani- mal is represented as divided horizontally, and in this horizontal section you see the cavity of the stomach forming one great whole in the centre and the partitions which extend from the external walls towards the centre either reaching the walls of the stomach or not, from the intervening septa. But these are not all equal. There are some of the partitions which reach half way towards the stom- ach—others which reach two-thirds of the wav— and others still which reach most of the distance. Below [Plate XXXII fig. B.] we see them as thev present themselvesiipon a vertical cut. From the external surface something of those partitions is already seen. The vertical striae noticed [Plate XX fig D ] are the external points of attachment of the fleshy partitions upon the external envelop of the whole body, and they-extend high up into the margin from which the tentacles arise. And indeed on close examination it will be seen that one tentacle arises always between two partitions; so that a tentacle is, as it were, a radiating prolon- gation of the main cavity of the body, extending like the finger of a glove from each of the divided spaces upwards. You see this [Plate XXXII flg. B.] where the tentacles show plainly their connec- tion with the main cavity, and where the divisions are as numerous as the tentacles. These partitions are muscular fibres, and by their contractions they can shorten the animal. Sup- pose these vertical partitions to be at once contrac- ted, the animal, instead of forming a vertical cylin- der, becomes depressed. [Plate XX fig. E J And as there are muscular fibres around the whole body, the tentacles can be drawn in, and the upper fibres, contracting more and more, may entirely conceal the tentacles and form such hemi spheri- cal bodies as are observed in these diagrams.— [Plate XX fig. G.J Between these partitions, by very careful investi- gation, small holes can be discovered, arranged in vertical series (fig. D). The use of these tubes is not yet fully ascertained. I shall have an oppor- tunity to refer to them again. But I would mention, further, that the mouth (Plate XX., fig. F) is not a simple circular hole on the summit of the animal, but presents lateral folds upon a longitudinal fissure. At first sight, when seen from above, the inner membrane of the Actinia stretched between the tentacles seems to form a circular mouth (Plate XXXJ.II, fig- A); but on close examination, it will be noticed that it is [Plate XXXIII.—Polypi—Young Actinls.1 40 PROF. AGASSIZS really a longitudinal fissure with lateral folds. All the tentacles terminate with a hole; they also con- stitute muscular tubes, with longitudinal and' cir- cular fibres, by the contraction of which they are alternately drawn in and out. The stomach, like the tentacles, empties into the main cavity of the body (Plate XXXII, fig. B), so that when the Acti- niae 8 wallows its food, the results of the digestion are thrown into this common cavity, and there circulates by the agency of vibrating cilia between the* partitions and in the hollow tentacles, until absorbed by the surfaces in contact. You see, also, in that diagram, that water can be introduced into the inner cavity through the mouth and the stom- ach, as well as through every tentacle, and also thrown out through stomach and mouth, and through every tentacle. The body is thus swollen by the water pumped through the suckers, or by that swallowed through the mouth. When the ani- mal re-opens its mouth to throw out water, the un- digested remains of the food are also expelled. When the animal comes out from its contracted position, we see the suckers gradually expanding, (Plate XX, fig. E) and these numerous tentacles pumping water, and the animal successively swell- ing into its virious movable changing forms. The existence of eyes in Polypi has been mentioned by Mr. Quartrefages. I have observed them in a new species of Lucernaria discovered upon the beach at Chelsea. In addition to these structures there is hanging from the partitions of the main cavity, [Plate XXXII. flg, B.] below the stomach, a series of bunches of eggs—ovaries, below those lower muscular partitions. All Polypi seem to have a structure similar to this. Those which do not re- semble these in structure, are the types which I consider not to belong strictly to the class of Poly- pi. When the eggs of Actiniae are matured, they are let out through the mouth. I have had an op- portunity to see this myself. These bunehes of eggs are freed in the main cavity of the body, and then through the lower opening of the stomach pressed into that cavity and finally discharged from the mouth, as represented in this figure. [Plate XXXIII. fig. A.] They are sometimes entangled in the cavities of the tentacles, and have even been reported by Sir John Dalyell to be discharged from the tentacles. The young egg of the Actinia presents the struc- ture which we observe universally throughout the whole animal kingdom. They consist of a mass of yolk substance, enclosed in a special membrane (Plate XXXIII, fig. B). Within is a germinative vesicle, and in the centre a germinative dot (fig. C). These yolks will grow (fig. D), the germinative ve- sicles and dots will disappear, and the germ being formed in the shape of spheroidal bodies, with a darker mass in the centre, will be batched, and form a more elongated body, (fig. E)—the yolk being more distinctly separated from the animal layer proper, which is the external crust of the germ,— Above, a depression is formed; the lower part is attached upon the soil, and around the upper depression, (Plate XXXIII, fig F.) there are little protuberances formed, (Fig. G > the central depres- sion growing deeper, and the mouth is finally pro- duced, surrounded by tentacles. (Fig. H-) But the most remarkable feature which I have observed la this development, is that the young Actinia differ from the old ones, in having at first only a few ex- ternal tentacles ; and these few are arranged in a very peculiar manner. Suppose this to be the first indication of the mouth ; there will soon be surrounding tentacles, (Plate XXXIII., fig. H.) at first only five, though in the full grown animal there will be hundreds. Next there will be others, coming out between the first ones, so that soon ten are formed. Then there are everywhere in the intervening spaces more coming out, so that twen- ty will occur; and in this way the number is grad- ually increased. But the position of the primitive five ones has a relation to the longitudinal form of the month; one of the five primitive ones being al way fin the same diameter of the animal as the longitudinal fissure of the mouth. (Plate XXXIIL fig. A.) But the other four are in pairs. After I had made this observation, I asked Mr Dana whether he had observed such a symmetry in the arrangement of the tentacles. He stated that he had; and that in addition, one of the tenta- cles was sometimes different in color from the oth- ers. What this means I shall soon show when comparing the Polypi with the other radiated an- imals. But now there are other Polypi whose embryol- ogy has been extensively studied. I mean the Co- rynse (Plate XX., fig. C.) Syncorynae (Fig. A.) and Podocorynae, (Fig. B.) upon which Loven, Sars Steerstrup, R. Wagner, and others, have made most remarkable observations. And also the Cam- panularia Tubularia, upon which we are indebted to Loven and Von Bereden, and others, for exten- sive information. The Coryne, and alike types are so closely related to the Tubulariae, that the resem- blance has been particularly noticed. And this close resemblance alluded to as a sufficient ground to leave the club-shaped Polypi with Medusa like buds among Polypi, notwithstanding the great differ- ence which has been noticed, both in their struc- ture and mode of development. Here we have the Podocorynte (Plate XX, flg. B), and here (Fig. C) the Syncorynae, which are small Polypi. The existence in Boston harbor of simi- lar Polypi of the genus Corynae, first described from the Northern Bhores of Europe, I have as- certained last year, and indeed there is a vast field to explore on these shores, as during a cruise on the South Shoals with Capt. Davis, in 1847,1 have ascertained the existence of not less than seven- teen species of this family, among which there are "types of new genera, which I shall describe on another occasion. From the upper part of the stem of these Corynoid Polypes there are hanging down several little bell-shaped bodies, of « quad- LECTURES ON EMBRYOLOGY. 41 ransular form. The outline of these bell shaped bodies being, when seen from below, as in figures A and B. The angles are prominent, and from them there are colored specks rising, similar to the eye specks of common Medusae. A membrane is stretched across over the central cavity, leaving. however, an opening below; and from the cor- ners are produced short tentacles, which, in the progress of time, grow longer and more moveable. In the interior there is a sucker-like projection, first with a single margin, which will be fringed afterwards. From these details it is plain that these buds, when fully developed, resemble most remarkably the small Medusa, (Plate XXVIII, fig. C) to which I have before referred. Indeed, they are finally freed from the stem upon which they grow, and move as independent animals. The structure of these small animals is indeed very simple; as they have only four straight tubes branching in four directions from their summit.— The investigators of these phenomena have been unwilling to refer them to the class of Medusae, but have considered them as closely allied to Tu- bulariae, and belonging therefore to the class oj Polypi. They have compared the Medusa-like buds of Coryne, Syncoryne and Podocoryne, (Plate XX.) to the crown of the Tubulariae, (Plate XXX, fig. A.) and you see that the comparison is very close. You see that the hollow tube within the Medusa-like bud (Plate XX, fig. A.) will com- pare to the hollow cavity with fringes hanging be low the tentacles of Tubulariae. (Plate XXX, fig. A.). Then you see the tentacles above spreading around the bunches of eggs and arising from the upper cavity, as the main cavity of the little Me- dusa-like buds surrounds its inner hollow tube, from which the eggs are developed in them, form- ing also special bunches, exterior to the inferior or anterior part of the alimentary canal, so that the resemblances between these bell-shaped bulbs (Plate XX, fig. F) and the crown of Tubulariae (Plate XXX, fig. A) is as close as it can be. The conclusion derived by Steerstrup from these facts is that the genera Syncoryne, Coryne and Podoco- ryne, (Plate XX, figs. A, B, C) should no longer be considered as genera by themselves, but only as the nurses'of animals of a higher order, the little Medusa-like animal, but that they nevertheless should remain with the Polypi near the Tubula- riae Steerstrup insists upon this point, when he says: "The more perfect forms, however, notwith- standing their resemblance to Medusae, must still occupy the systematic place of the clariform Po- lypes, or Coryne, as animals closely allied to Tubu- lariae, Sertularia, &c. &c. ' Let us now examine the Tubulariae and also the Campanulariae, as they have been carefully studied, and then we shall be prepared for an opinion upon these conclusions. We have here [Plate XXVUI] a stem of the Campanulariae, which has branches of various kinds. How these branches grow must b« flxamined more fully. [Plate XXVIII—Campanulari.e.J In a growing stem—the first origin of the stem we shall examine afterwards—there is in the inte- rior a cavity, which cavity expands above and forms a kind of stomach; the moveable part of the animal forming tentacles around, and the mouth being therefore above. And from the side of such a Polype there will be, after a certain time, a bud, forming a simple sac, communicating with the main cavity, and the changes which have pro- duced the main stem will be repeated here so as to give rise to another Polype of the same structure as the terminal one, with a open communication with its main cavity ; and after by repeated budding, numerous branches, all alike, have been found as they are figured in this diagram, [Plate XXVUI] Where you see seven buds all alike.some new buds forming in the axis between the main stem and the first buds. And these new buds differ from the former, inasmuch as the bud will not terminate with a new Poly pe.similar to those of the first buds, but will remain closed, and while it is still closed there may be buds arising on its side in which eggs are developed. Loven, who described these phenomena more extensively, represents these axilary buds as giv- ing rise, by budding, to new branches, remaining longer shut in a common cavity, and indeed being branches similar to the external one; with the only differences that the terminating animais have smaller tentacles, and are of a slightly different shape; communicating with the main cavity, and giving finally rise to free moving individuals; whilst there are below simpler sacs, of the same order, but still less developed. Plate XXXV rep- resents the various stages of this growth. Now these sacs are something like buds; but they are, in fact, eggs, which, in the beginning, are simple buds, or diverticula from the common cavity, so that we can consider the whole as buds, which throw out new buds, from which eggs are developed, in the shape of pouches. And that these are eggs, can be proved by the characters which distinguish eggs. (PI. XXXV. fig. D.) They may have a germinative vesicle, and a germinative dot; and there a new animal is formed, which will escape as soon as the upper buds, which are j now full grown, have removed the closing opercu- ! lum: so that, by a process of budding—of bud- ding egg like buds—there is a new generation, formed, which does not remain upon the primitive j stem, but is freed; and when freed, the germs arising from the eggs are elongated, and little cylindrical animals, which swim free, appear; and 42 PROF. AGASS1Z S [Platp. XXXV—Bnnmvr, or Oampanularia.] after having continued free for a certain time, they become attached, and then the whole mass is de- pressed and enlarged into a disc-like body, the centre of which is somewhat prominent; rises then more and more, and begins to be transformed into a little stem; and this little stem will open above, and form a termination, like that of the common buds of Campanulariae (Plate XXVIII.) that is to say, an animal with an internal cavity— We have thus again a beginning of one of those complicated stems, which, by the multiplication of their buds, form communities of animals, of two kinds, viz: such as are individuals similar to the animal at the end of the main stem, and others from which a free generation is produced, and which, after remaining free for a certain time, go on to repeat the same process of branching and budding. In the Tubulariae (Plate XXX, Gg. A) we have a similar growth. One of these bunches of eggs, when examined in its immature condition—in its earliest formation, (Plate XXX, fig. F)—is simply a branch with lateral buds, and the digestive cav- ity communicates freely with all these little buds. But their interior mass assumes gradually a more rounded form, and is fuccessively enclosed in the external mass, which will enlarge, and then there will be finally isolated eggs developed, in the form of bunches, when upon the summit of every one of them a distinct animal envelop is formed which extends downwards upon the yolk—as the internal mass can be considered as a yolk—and after it has grown so as to appear like a cup. with tentacle-like appendage1?, the little animal is freed, and has a structure like the young Medu«a. as It is figured from a Campanularia, in Plate XXXV, figures T, P and Q. The whole process of budding in this animal is shown in figures A, B, C, &c, (Plate XXXV)-first, the changes which regular common buds undergo in their development, and next, (Fig. E to G), the changes of the eggs prop- er, with their animal envelop surrounding the yolk, and finally dividing into tentacle-like appen- dages below. The internal cavity being formed by the changes which the remnant of the yolk undergoes. The young animals which are derived in this way in Tubulariae and Campanulariae, from egg bunches, are so similar to the free buds from Corynae, Podocorynae and Syncorynae,(Plate XX, figures A, B and C), that their analogy cannot be mistaken. This resemblance can even be recog- nized in stages of growth not further advanced than these, (Plate XXXV, figs. T, P, Q). Some Medusae occurring on these shores—for instance the genus StomabracCium—have a very close re- semblance to those germs of the Campanulariae (Plate XXVIII), and Medusae, with only four arms and four tubes diverging from the central cavity, with fringes all round: and I should not be sur- prised at all, if Stomabrachium was finally found to be the free Medusae-like generation of Campa- nulariae. But now as the affinity between all these Polypi (Plate XX, XXVIII and XXXV) and the Tubulariae (Plate XXX) is very clearly shown, and as on that account these animals are all con- sidered as belonging to the class of Polypi, though they give rise to animals so closely allied to Medu- sae, the question arises how far Tubulariae itself can be considered as strictly belonging to the class of Polypi, of whether it would not be more nat- ural to view it as a type of Medusae, furnished with a permanent stem. The only objection to this is, that true Medusae are not formed in the same way as Medusa like free buds of Coryne, Podocoryne and Syncoryne (PlateXX.) These have arisen from buds grow- ing upon Polypi-like stems, though they are final- ly Medusae-like animals; whilst true Medusae are multiplied by transverse division of Polypi-like stems, which can have no influence upon our ap- preciation of their real structure; so that the ques- tion properly is, whether there can be real Medusa) with a stem, or not. We have, therefore, in this stage of the investigation, before deciding one way or another, to compare the true Medusae (Plate XXXVII and Plate XXVIII, fig. C) with those Polypi, the Tubularia, (Plate XXX), when it will be seen that their structure agrees in every respect vbut that one, that the Tubularia, with its crown rests upon a stem, whilst Medusae proper are en' tirely free. The great difference there seems to be in the forms of these animals is more apparent than real, the cavity which hangs below the ten- LECTURES ON EMBRYOLOGY. 43 tacles corresponding to the central alimentary tube of the Medusae, which is only drawn in be- tween the gelatinous walls of the disc, though it remains equally free as in Tubularia. The upper cavity of Tubularia answers to the disc of Medusae proper with its cavities; and in both the ovaries are outside of the alimentary cavity, as well as of the main cavity of the body Indeed, the agree- ment is perfect in every respect, and we must come to the conclusion that from their structure Medusas and Tubulariae must belong to the same class, Tubularia being Medusae with a stem, and bearing the same relation to free Madusse. as cri- noids bear to free starfishes. And so we have in the class of Medusae attached types, as well as in that of Echinoderms, and in that of Polypi. In a more general point of view, we may, how- ever, compare further, ail radiated animals, when we shall find that they really constitute a natural, well circumscribed group in the animal kingdom agreeing in all important points of their structure being strictly constructed upon the same plan, al- though the three classes which we refer to this great department differ in the manner in which the plan is carried out. In the first place, I may mention that besides Polypi, Medusae and Echi- noderms, the other classes which were referred to the type of Radiata, have been removed from it, or are to be removed from this connection. The intestinal worms indeed are truly articulated ani- mals in tbeir fundamental plan of structure,'and have to be connected with the worms proper, while the Infusoria, Polygastrica and Rotatoria are very heterogeneous classes, the latter of which has to be united with the Crustacea and the so- called Polygastrica, to be divided off according to their various structures, some being germs of aquatic plants, and others the first stages of growth of various worms, as I have ascertained by direct observation. As for the classes of Polypi, Medusae and Echinoderms, if we bring together the dia- grams (Plate XXXII) representing an Actinia in a vertical section, with that of Plate XXXVII, which represents a similar section of a Medusas, [Plate XXXVII—Medusa.1 vert the Polype and place it with the month downwards, as it is naturally in free Mec!u-ae, we could see at once that in the Polypus we fPf,»TP XXXVTTr UK„Tv(n,rn«"r«r and other illustrations of Echinoderms exhibited in a former lecture, and the vertical section of an Echinarachnius, we shall have the elements (Plate XXXVIII, fig. E) of a closer comparison be- tween the three classes. If we were indeed to in- have the same ifeiierai arrangement as lu me .>le- du^sv. There being a separate alimentary cavity and a common cavity of the bpdy only in Medu- sae, (Plate XXXVII) the anterior part of the ali- mentary cavity hangs down with the mouth freely from the walls of the body. This part of the ali- mentary canal answers to the cavity of Actinia (Plate XXXII, fig B) which is called stomach, and from the upper part of the Actinia, in its inverted position, arise those partitions which end in ten- tacles answering to the disc of Medusae, with its cavity, branching into similar tentacles. We have also again a common cavity in Medu- sae (Plate XXXVII), as well as in Actiniae, only more circumscribed, and branching off into tubes which communicates in similar mannerwith the ten- tacles, so that the general arrangement is perfectly identical. The difference is, however, this—that in Medusae the tubes arising from the central cavity are circumscribed, while in the Actiniae (Plate XXXII, fig. B) they are only partitions communi- cating all together. And in the Medusae (Plate XXXVII) there is a distinct nervous system. I suspect that in Polypes we should find the nervous system in the same position as a ring round the mouth, if it is at all distinct in those animals; that however eye-like specks have been noticed, even in these lowest animals, I have already mentioned. As for the ovaries of the Medusae (Plate XXXVII), 44 PROF. AGASSIZ S they arise externally from the lower or central ca vities of the alimentary canal, and are surrounded by the disc, which contains the main cavityof the body, and from the periphery of which the tenta- cles hang down, so that here the ovaries are out side of the stomach, and outside of the main cav- ity, as in Tubularie,—and not within the common cavity, as in Polypi. Now, in order to insist more strongly upon the fundamental differences which exist between Polypi and Medusae, even if we include Tubulariae among the latter, let me once more call your attention to the Tubularia (Plate XXX, flg. A). We have here a mouth, with the anterior alimentary cavity, which will assume all possible shapes, as we see in these various diagrams, hanging outside of the common cavity, and not within it, as in Polypi. We see those bunches of eggs, arising below the tentacles, between the tentacles and the anterior alimentary cavity, also outside of the alimentary cavity, the central cavity extending above, so that the analogy is perfect in every respect. And as we have in the Corynae, Syncorynae and Podocorynae buds, which, though growing from Polype-like animals, will produce real Medusae, their close resemblance to Tubulariae will only be an additional evidence that these mast be referred to the class of Jelly-fishes, and that the club- shaped Polypi, in their perfect condition, are also Medusae, and that their earlier stages of growth are only nurses to produce real Medusae by alter- nate generation. The Tubulariae themselves will have, however, to be considered as the lowest type of Medusae, preserving something of the Po- lype structure, as they are for life provided with a stem, from which the crown hangs down. And from this stem would arise buds similar to the ter- minal animal (Plate XXX, flg. G) which would remain connected with the stem, thus forming branched compound Medusae. And if this ground be correct, not only Tubularia, but also Campanu- laria and Sertularia shall be united with Corynae, Syncorynae and Podocorynae in the class of Medu- sae Thus circumscribed, the class of Medusae would present the most remarkable parallelism with the class of Echinoderms and that of Polypi, in both of which there are free types and such as rest upon attached stems, a parallelism upon which Oker has already insisted, in a general way, is his classification of the animal kingdom. To investigate further this subject, there is a rich field in this vicinity, where animals, Tubularia, Companularia and Sertularia, occur all around the shores of Massachusetts. Again, if my conjecture of the necessity of com- bining these Tubularia with Medusa is correct, I venture to foretell, that among those small species of this class, which are found on this shore, we have the Medusa-like form of the Coryna, in the little Oceania of Dr. Gould's Report, whose struc- ture is illustrated in Plate XXVIII, fig. C., as I have ascertained by dredging, that Coryna occurs in Boston and harbor. That Coryna has been found so seldom is be* cause it lives in deep water, and is not discovered urvtess by dredging. I should not be surprised at all to find also Stomabrachium, as the Medusa-form of Campan* ularia, which occur all over the shores of this con- tinent, and that Bongainvillia could be the Medusa of Tubularia, if they produce at all a free genera- tion, seems to be probable, when we consider the form of its crown. (Plate XXX.. fig. A.) As for the gradation of types in the class of Me* dusa, we should consider the Tubulariae as the lowest, for the reasons already stated. Next we should place the free compound Medusae, the Phy* sophorae of Eschscholtz, which correspond to the next stage of growth of Medusa, known under the name of Strobila. [Plate XXVI—Medura [ Next we should place the free Medusae or Disco phora of Eschscholtz, and highest the Ctenopboras as by their comb-like rows of fringes, which may be considered as a lower form of Ambulacra they LECTURES ON EMBRYOLOGY. 43 come nearest to the Echinoderms. This arrange ment, which is natural in itself, would show the most admirable agreement between classification and the phases of embryonic growth in this class, and also because they come nearest to the first staee of growth of the common Medusa. (Plate XIV and XIX] [See Plates XIV Lecture 3, and XIX Lec TURK 4 ] The analogy again between Medusae and Echi- noderms is too easily ascertained to be ever mista- ken by any one who attempts to compare them in the same close manner. The chief difference here consists in the more developed inner structure of Echinoderms, whose organs are more diversified and isolated, and in the harder coverings which protect the soft parts, besides the addition of some special apparatus which do not occur in the two lower classes of Radiata. The improvements which I anticipate in the class of Polypi are fewer, after removing the Retepora and alied types, to the great groups of Mollusea and the Tubulariae to the class of Medusa. We shall only introduce the Porpite and Velella in the vicinity of Actinia,and then, as Jlr.Dana.ha8 done, 6 divide the Polypi proper in Actinoids and Alegon- oids, the former division embracing those with simple tentacles, as Actinie, (Plate XX fig. D.) the latter those with fringed tantacles as Alegorium and Renilla. (Plate XXXL) All the stone corals proper belong to the type of Actinia, and upon a close comparison of the struc- ture of this animal with the ancient fossil Cyntho- phyllum-like Polypi of palseofoic rocks, some fur- ther hints may be derived as to the order of sue cession of Polypi in geological times, which is at present very little understood. How the calcare- ous stem is formed in Polyps, can be perhaps no- where better studied than in the little Alcyoniura (Pate XXXI, figs. A, B,) of Boston harbor, where calcareous nets and spicules are deposited in regu- lar groups below and within the base of the tenta- cles, and at the opposite extremities of the animal, between which the muscular fibres are attached.— There is, moreover, a peculiarity in the structure of Polypi, which can be easily observed in the Ranella. (Plate XXXI, fig. E.) In this Polype the mouth has an elongated form, and there is one tentacle in advance and one behind this opening, in the longitudinal diameter of that fissure. Under the form of radiated animals we have, in- deed, through the classes of Echn^Jerms, Medusae and Polypi, every where indicatWis of a bilateral symmetry, concealed under the more prominent outlines of a radiated arrangement of the parts.— We have really among Radiata the first indica- tions of the general bilateral symmetry which pre- vails universally throughout the animal kingdom, even in the class of Polypi. (Plate XXXIII, fig. A.) In Actinia, the lowest condition, this bilateral symmetry is noticed in the longitudinal direction of the mouth, (Plate XX fig-F) and in the ar- rangement of the first formed tentacles, of which one is seen always in the same diameter with the mouth, whilst the other tentacles are placed in two pairs on each side, (Plate XXXIII, fig. H) which is peculiar in such species. We have also indications of a bilateral arrangement in those Medusae in which the body is compressed laterally and more or less oblong, as in Beroe, Cestum, &c. where one diameter is much longer than the other. We have it still further in the division of the ten- tacles hanging down from the mouth in the com- mon Medusae, in which there is frequently one tentacle more developed than the others. That Echinoderms are regularly bilateral under their spherical forms, I have already shown, fifteen years ago, when I first ascertained that the Ma- dreporia bodies lie always symmetrically between two of their rays in the longitudinal axis, which it parallel to the direction of the alimentary canal, as it extends towards the elongated extremities of the higher types of that class. Another peculiar arrangement which is common to the Radiata, is the existence of water tubes, es- tablishing a permanent connexion between the surrounding element and the internal cavity of the body. In the Medusas (Plate XXVII, figs. A 46 PROF. AGASSIZ'S and B), I have already shown the structure by which the waiter is introduced into the cavity. In the Echinoderms is figured this arrangement in the star-fishes (Plate XXXVIII, fig. D). Through these almost microscopic tubes the main cavity is constantly filled with water, which escapes freely from the star-fishes when they are taken out of the water. They should not be mis- taken for ambuloeral tubes, which are placed in re- gular rows—whilst the water tubes are scattered almost over the whole surface of the animal, but only seen when fully expanded in the living ani- mal. In the Actinia, the water system is plainly developed (Plate XX, fig. D), in the forms of mi nute pores arranged in vertical series. -From the above statements it can be concluded, that there is the strictest agreement between all Radiata in the general plan of their structure; and this analogy can even be traced in the embryonic growth—all the Radiata beginning by the formation of a distinct layer round the yolk in the form of a spherical crust, from which the more animated parts are derived, whilst the alimentary cavity is formed by the modification of the central mass of yolk. In addition to this regular mode of repro- duction, the Polypi and Medusae are also multiplied by buds, and sojoe of the Medusae by a peculiar modification ofwe alternate generation—new ind> viduals being formed by the transverse division of a primitively simple stem. Whether anything like an alternate generation takes place in the class of Echinoderms, remains still doubtful; but I cannot help thinking that the Pedicellariae are the last in- dications of a kind of budding, giving rise to very low organisms, which can only be compared to the peculiar beak-like buds of some of the Sertulariae. This uniformity of structure and growth calls for an additional remark. Ever since the natural and physical sciences of graphical representations have been introduced, progress has been made much more rapidly than before. As soon as Humboldt bad drawn bis isothermal lines, investigations in all parts of the globe were at once called for. And so it was in chemis- try, when the formulae were introduced to re- present chemical composition, by which an insight into the constitution of numerous bodies could be obtained at one single look. Now in the animal kingdom nothing has yet been done to represent by symbols either structures or natural affinities; only the teeth of Mammalia are noticed in a regular system. Something, however, has been done, and is extensively introduced in Botany, to represent the arrangement of the leaves of plants and the parts of the flowers, by formulae. But to represent structures—to represent affinities by symbols is an attempt which has not yet been made, and which I think could now be satisfactorily introduced. Only general symbols for the main groups of the animal kingdom, representing their fundamental embry- ological character, have been introduced into the text book which I have published in connection with Dr. Gould, where a star was used to represent the Radiata, where Mollusca when represented by an inverted Greek W, Articulata by a W, and Vertebrata by the figure 8, these diagrams having reference to the peculiar mode of development and of the germ. That the Radiata is best represented by a circle, is shown by what I have said of the first formation of the germ, which surrouuds the yolk entirely from beginning, and forms, as it were, an animal crust round the yolk, so^ that we could have, instead of'a star to represent'Radiata, any general simple circular outlines with a dot in the centre, to remember the analogy of their gen- eral structure with that of the eggs, with the low- estcondition of all animals. [Plate XXXIX ] _____ But when we would like to represent special classes, either,. Polypi, Medusae or Echinoderms, 1 would propose that instead of a dot, we should have for the Polypi a longitudinal line across the circle. (Fig. B.> indicating the first apperance of a bilateral arrangement under the form of a sphe- rical cirele. To represent the Maduste, I would propose a circle with a cross within, (Fig. C,) to indicate that in these animals there is a radia- tion of branching tubes from the central cavity. And to represent Echinoderms, I would have a star in the circle, (Fig. D.) corresponding to the form which is the most characteristic of that class. So that the three classes of Radiata would be represented by their peculiar figures, and by the addition of a single letter to these symbols, we might at once represent either of their families— for instance, having the diagram of Echinoderms, an additionrl C would represent Crinoids, E would Indicate Echini, and A would represent Asterid* (Fig. E). And how important this would be, is at once ob- vious, if we look at geological works, where the lists of fossils, simply mentioned by their names,do not convey any idea to the reader. But if, instead of Saccocoma, shortly we append the figure of Echinoderms, and add aC, we should know at first sight that this is a fossil of the class of Echino- derms belonging to the family of Crinoids,and the symbol itself would at once remind us of the pecu- liar structure of these animals. Those great fig- ures being used to indicate the families, an addi- tional small letter might indicate minor divisions, and so on; so that these symbols would show all the affinity of any given animal, and form in real- ity a complete picture of the various relations which exist among all animals. In my next Lecture, I shall enter into the depart- ment of articulated animals. LECTURES ON EMBRYOLOGY. 47 LECTURE VI, t now proceed to examine the great group of the animal kingdom, which Naturalists have desig- nated under the name of Articulata. These ani- mals are remaikable for one peculiar feature of their structure; the body consisting of a series of joints moveable upon each other, to which are fre- quently added moveable appendages, sometimes subdivided into joints, which are moveable also. This is the common character of all Articulata, and upon Plates IV, V, VI, VII, IX, X and XI you see various forms of this great type. [Plate IV—Rat-taileo Worms 1 '[Plate VI—Lobster] The Articulata have been divided into three classes: Crustacea, as crabs, lobsters artd all the animals like them; Insects, as butterflies, beetles, flies; and Worms, the worms which live free in the water or in the soil, and also the parasitic and intestinal worms. These three classes differ in their structure as well as in their general form, and they have been placed in our svstemaiic works in an order which deserves particularly to attract our attention. The Crustacea are placed highest in the series of Articulata, and the Worms lowest; and between them, the Insects, so numerous and so exceeding- ly diversified. In the opinion of Naturalists, this order of succession agrees with the complication in structure of these animals. And they insist upon this order as really indicating the natural gradation among them; the Crustacea being con- sidered highest, owing to the perfect development of a heart and a regular circulation, and also owing to the concentration of the nervous system and the combination of its elements. The want of a regular circulation in the Insects has been the reason why they have been placed in the second rank. The Worms, from the uniformity and num- ber of their rings, to which are attached feet-like appendages almost as numerous as the rings them- selves, have been considered as the lowest. Now in this order of succession, to which Natu- ralists have specially devoted their attention, which they have investigated with particular refer- ence to a natural classification, I think we have another instance of a mistaken view of the sub- ject, derived from a pn'staken estimation of an- atomical characters. I am prepared to show that Crustacea are not the highest; that Insects should be placet} at the head of Articulata; and that they are in every respect the highest. And after the grounds upon which I intend to place them high- est have been illustrated, I expect it will be found that the anatomical structure agrees here again with the order which the metamorphoses actually indicate; and that it was a mistaken view of the complicated structure of the Crustacea which in- fluenced Anatomists, and induced them to place Crustacea highest. Before, however, I can go through this compar- son. I must illustrate in detail the different classes of this great group; otherwise my comparisons and my grounds would scarcely be intelligible. I shall devote this evening to the illustration of that one class which I consider as highest among 48 PROF. AGASS1Z'V (Platb VUI—Scorpion. Articulata—that of Insects. And before begin- ning this investigation, I will simply mention that the group of Articulata, as it is now circumscribed, has not always been considered as containing only three classes. A great number of divisions and other arrangements were, at various periods, at- tempted by Naturalists, The Spiders, for instance, were considered as one entirely distinct class, placed between Crustacea and Insects, though I am of opinion that they are better united with the Insects, owing to their structure, as well as their natural development. Among ArticuJata, groups have been introduced, which were formerly placed in other great divi- sions. For Instance, the Barnacles were long con- sidered as Shells, from their external coverings, which are really shells -, but their anatomical struc- ture has proved a relation between them and Artic- ulate animals, and really a close relation to Crusta- cea proper—so close a relation to Crabs and Lob- sters, that, at the present time, no Anatomist doubts that the Barnacles must be placed in one and the same class with them; though perhaps among Zoologists, there may be some who still think that the external form should be taken into considera- tion, and not overruled by the internal structure-, but such doubts deserve scarcely any longer no- lice. As I mentioned in the last lecture, intestinal worms were placed among Radiata, but they are proved to be Articulata, since the nervo»s system tins been lately discovered by Mr. Bianchard in all the principal types of intestinal worms, and found to agree,bu: with some modifications in its genera? arrangement, with that oT' Articulata. The Infuso- ria were also formerly arranged among the Radi- ata, but now their structure is more extensively known, they should be scattered and arranged among various classes, according to their inner organization and mode of growth—some belong- ing to the Worms, and being only the young, or embryonic condition of worms of Planariac, for in- stance ; others belonging to the vegetable kingdom, and being also embryonic conditions of various1 Algse-; and others still, belonging to the Crustacea, as for instance the Rotrfera. It is remarkable that the extensive investigations made upon the Infu- soria, the object of which was to illustrate the uni- form structure of these animals as a class, go to show that the class ought to be broken up as a na- tural group, and distributed among various other classes. How much remains to be done among the small organized beirrgs.which have to be investigated by the microscope, will be at onee understood when 1 mention that, for instance, the egg of the Mosquito- like animals whose embryonic changes are repre- sented in Plate VII, figs. A. and F.,was first consid- ered as an Alga.and described as a species of Gloc- onema, before it was found to be a Musquito-like insect. [Plate VII.—Eggs of Musquito s] The great class of Insects is particularly remark- able for the metamorphoses which these animals undergo. And you may at once perceive how dif- ficult it must be to trace all the changes of these animals when I mention, that the perfect being— the perfect insect may be an aerial animal, provided with wings, and flying about; when in another con- dition, it is quietly buried in the soil, immovable, not taking any food : or, in another condition, it in an aquatic worm, swimming freely m the water. Under sueh circumstances, unless there is an op- portunity to trace all these successive changes, you see how mistakes, as gross as the one to which I have alluded, may be made. Naturalists are now aware of the possibility of such mistakes, and do not consider an investigation as perfect, as long as the direct connection between the facts in any giv- en case has not been ascertained by continuou observations. Articulata undergoing such exten- sive changes, sn»«., therefore, be studied i» waay LECTURES ON EMBRYOLOGY. 49 more points of view, under more manifold aspects, than any other animals. And we have here to in- vestigate external changes, as well as internal mod- ifications of structures; changes of habits, as well as ch,anges of forms; indeed all the successive trans- formations through which these animals gradually pass from their formation in the egg to their perfect condition. The embryology of Insects proper has not been so extensively and so fully studied as the embryol- ogy of other classes. There is generally a great difficulty in examining the eggs of insects, owing to the opaque condition of the yolk-substance, the softness and transparencv of the primitive germ, and the thickness of the horny envelope which surrounds the e_cg. You see under what difficult circumstances the observer is placed, to have to break up this hard crust without injuring the soft and delicate germ—which is, besides, exceedingly small,—and then to distinguish the various forms of their transparent body, resting upon a dark.opaque centre;—circumstances the most difficult for micro- scopic investigation which can be found. And so we have only a few species whose embryonic growth has been satisfactorily examined. Professor Kolliker of Zurich, has made those investigations, and I introduce here, (Plate VII.) the diagrams which he has published of one of those series, in 6rder to show how peculiar the mode of growth of insects is, and how different it is from the changes which other animals undergo within the egg. , After tracing those changes which take place within the egg, I shall proceed to allude to the changes which the Worm undergoes to form a per- fect Insect. The egg itself consists universally among all insects, of a yolk of opaque substance, enclosed in a hard envelope. When the eggs are laid, there is no germinative vesicle, no germina- tive dot, seen within. The eggs have really un- dergone extensive changes before they are laid, and when laid, the envelope which surrounds them is already thick and opaque. In order to as- certain whether the egg has primitively the same structure as that of other animals throughout the animal kingdom, it Is necessary to trace the for- mation of their substance back to the ovary, and examine the young egg, when the germinative vesicle, with the germinative dot, surrounded by a transparent mass of yolk, enclosed in a mem- brane, will be observed, as in all animals; and it is only shortly before the egg is laid that a thicker envelope is formed by the addition of layers of more consistent matter, which are successively de- posited in the oviduct around the yolk membrane, to protect more effectually the eggs, which in so many insects have to pass the winter in that con- dition, before the caterpillar or worm is hatched. However, in the investigation of the formation of the egg and its envelopes, there remains much to be done in the class of Insects. It is a peculiarity with the eggs of insects that they remain a long time after they are laid, before undergoing their regular transformation ; at least, this is the general impression. That, however, regular transformations begin in the winter, and go on during the cold season in this well-protected cuirass, has recently been ascertained by a gen- tleman of this city, Mr. Waldo I. Burnett, who is at present in vestigating successfully this diffi- cult subject; so that the changes taking place in the eggs of various insects are likely to be soon supplied. [Plate X.—Insects with their Larvae and PupjE ] The form of the eggs of Insects is exceedingly variable. There are eggs, for instance, which are attached to a long stem, (Plate X, fig. B) from which they hang down. That stem, however, be- longs.not to the epg proper, but is only a part of its external covering. The layers of protecting substance around the egg,are extended beyond the growth of tie egg itself; and through these stems the eggs are attached to leaves of trees, resem- bling little fungi or cryptogamic plants, for which they have been sometimes mistaken. The first thing which takes place in the egg after the ger- minative dot and germinative vesicle are gone— after the yolk has become opaque, is the forma- tion of a transparent layer of substance all around the yolk, as seen in Plate VII, fig. A, which repre- sents the young animal, or gcrm.in its earliest con- dition. As soon as this animal coating has grown 50 PROF. AGASSIZ'S sufficiently thick to assume definite outlines, a broad open space is noticed on one side of the germ, through which the yolk is very extensively seen. From further changes, it will be ascertained that the continuous mass represents the ventral portion of the animal, and that the free opening is on the dorsal side of the germ. At this earliest stage^some few changes oi substance have already taken place. The animal layer, when first formed and examined under the microscope, is seen to consist of small cells, which have little dots with- in. At first, there is only one layer of such cells ; then,a second layer is formed.probably derived from the substance of the yolk itself. Then there are three or four such layers, the cells being probably multiplied and increased in number by the burst- ing of the primitive cells, and by the growing into cells of their minute inner dots. This seems the more probable, as with the in- crease of Iayers,the cells becoming more numerous, are also found to be smaller ; so that, when there arc four or five such layers, the cells are so minute as to require a higher power of the microscope to examine them ; showing that these cells increase by evolution from the primitive ones. The ap- pearance of a thick animal layer around the yolk, as the first indication of the eerm, with a large open space opposite the main bulk of the embryo, is a peculiar feature of the mode of formation of in- sects, by which they differ widely from other ani- mals. Here, (Plate VII, fig B) the opening is to- wards one end of the egg. at which end we also no- tice upon one side_of the germ, the first indication of a transverse division, marking out the head.— Next, (Plate VII, fig. C,) there will be some con- tractions taking place upon the longitudinal axis of the body, dividing the germ into several joints. The first change which takes place in the germ of an articulated animal is, therefore, an indication of the type to which it belongs. It is realiy an artic- ulated animal before any further indications of a structure are introduced. The first division which takes place goes to indicate the position of the head. At this period, (Plate VII. flg. B), the yolk mass is already reduced to a smaller space. Next the transverse divisions appear, those of the head growing more complicated as -represented in Plate VII, figs. C, G, H. And then, there is a well defined outline formed below the yolk, (Fig. D) extending to the anterior divisions of the germ, and towards its upper side, going to form the alimentary canal. The mass of the yolk is still more reduced, the membrane which now encloses it from below hav- ing folded itself upwards, so as to assume the shape of a little boat, (Fig. E,) and parcels of yolk re- maining scattered on the sides. At this period we can already observe that the folds on the outside of the body will be transformed into joints. There is a head at the upper end of the germ, and at its lower side there are indications of legs (Fig. E). A wonderful arrangement is now plain, which was first discovered among Articulata by Herold, in Spiders, and afterwards confirmed by Rathke in Crawfishes, namely, that in articulated animals the folding of the germ takes place in such a manner as to have the navel upon the back, that is to say, the opening by which the mass of yolk communi- cates with the alimentary cavity has a position strictly opposite to what is observed in other ani- mals. The germ, indeed, folds itself around the yolk, leaving a broad opening on that side of the animal which, in its final structure, will be the back. (PlateVII,Fig. D.) The side opposite the na- vel being the one from whence the feet come out, and that where the opening is observed, being the sideLfrom which the wings will be developed. The membrane which was developed below the yolk has now folded itself more extensively upward, and forms an elongated open channel, which finally grows into a closed tube, the alimentary canal, as it is seen in the animal more fully developed (Fig. F), where there are some parts of the yolk remain- ing in the joints. Before the yolk has entirely disappeared, there is a pair of rudimentary feet developed in the anterior part of the embryo, which will disappear before this embryonic ani- mal has the proper form of the larva to which it gives rise. There are also at the posterior extrem- ity indications of false feet forming, and all along the various joints of the body, which have been successively marked. These are, however, not feet proper, but only stiff hairs. From the facts stated above, it is plain that in the class of Insects, after a complete investigation of the growth of the egg of one species, (and indeed of several species) it has been ascertained that the germ is not developed above the yolk, but below, as we have observed it in Radiata There is not, as in Radiata, a cavity formed below, extending with- in the bodv to the stomach and the mouth; but we have in this case a germ which is forming below the yolk. Of course, such an egg could be re- versed, and it might be said that there is no differ- ence between the germ of Radiata and Insects — that we may just as well turn the egg of Insects so as to have the germ in the same apparent position in both cases. But if we turn in such a manner an egg of an insect, with its germ, we shall find the feet growing out of the upper side, and we shall find the opposite, or lower side, giving rise to a pair of wings. This would only show a re- versed position of the whole; as we may place the feet of an Insect upwards, and the wings downwards, and have only an inverted Insect. But by thus changing the external position of the animal, the legs remaining opposite the wings, whether the navel be primitively open between the wings or above the animal, or vice versa, we shall not change the relation of its parts, in their growth. And so you see, that the articulated animals grow in a position the reverse to that of the Radiata, and undergo successive changes, which at a very early period give rise to those moveable joints which characterize Articu- LECTURES ON EMBRYOLOGY. 51 lata in general, and are seen in the lowest forms, as well as in the Lobsters (Plate VI), or Scorpions (Plate VIII), or any of the insects. That this mode of growth is not peculiar to in- Bects alone, but is characteristic of Articulata at large, follows, from the beautiful investigations of the embryonic growth of Crustacea and Spiders, which have been traced by many Naturalists, but above all by Herold, Rathke, Erdl, &c. That the same mode of growth is also observed in Crustacea and Spiders, can be satisfactorily as- certained by a glance at plate IIL where in a Shrimp the germ is seen developing below the yolk. TPlate III—Young Shrimps. | The details of these metamorphoses I shall illus- trate thereafter. I mention it now, only in or- der to add, that this mode of growth is not pecu liar to insects alone, but that it is characteristic of most Articulata to have this inverted mode of growth from their earliest embryonic condition.-^ They grow, as it were, in opposition to all other animals. And it is a fact in no small degree re- markable, that among such animals there should be such a number of Parasites. Articulata are, however, the only type in the animal kingdom in h ch parasitism is the prevailing rule, though there are other Parasites which belong to other classes. The metamorphoses of Insects which take place after the little Larva (as Entomologists call the earlier condition of the animal) is born, have been extensively studied. This, little Worm (Plate VII, fig. F) is like the primitive form of the com mon Mosquito, of which we see (Plate IX, figs. B, B, C) all the different changes which the animal undergoes before it is changed into its perfect state. Figs. D, E, F represent the same successive changes from the Horsefly ((Estrus); figs. G, H, I, those of the common Flea; figs. J, K, L, M, those of the Cochineal. In plate X, the figs. A, B. C represent the egg, larva and perfect Hemerobius ; figs. D, E, F the metamorphoses of a Moth, of the genus Geometra; figs. G, H, I those of Phryganea, and figs. J, K. L those of an Ephemera; plate XI represents Beetles; figs. A, B, C the metamorpho- ses of a Dermestes, whose larva is hairy and col- ored, like that of a^utterfly; and figs. D, E, F, that of a Cetonia, in which the larva is a Maggot. The Naturalists of the last century have studied, more carefully and more extensively the metamor- phoses of insects than the Entomologists of the present day. It is to works long since almost for- gotten among entomologists, that we must resort to fiud extensive, minute, and correct information upon the metamorphoses of Insects in their vari- ous stages of growth. Swammerdam, in his Bible of Nature, full of interesting details, has given a great variety of metamorphoses. So have the investiga- tions by Degeer, Geoffroy, and Rosel, done more in this department than all modern investigatiors put together. [Plate IX—Metamorphoses of the Mosqui- to, Horsefly. Flea and Cochineal. The title of Rosel's work, which he styles "Amusements with Insects,"(/nseten Belustigunyen) shows how much he must have enjoyed his re- searches. He has, perhaps, illustrated the meta- morphoses of insects more fully than they have been examined before or since. In our modern times, Entomologists have devoted almost all their attention to the study of genera and species, of the external forms of families and specific distinctions, and have in this way, endowed Entomology with treasures of detail, but have made very few refer- ences to the study of metamorphoses, which would however, render this minute knowledge cf details much more valuable; for if the changes which take place in various families were brought under rules, these details would at once be made useful in the comparison of extensive series. But, for the pres- ent, we have only to hope for a general comparison between the modifications of parts as they occur 52 PROF. AGASSIZ'S in the larva state, with those of perfect insects. I would, however, except from this criticism some few modern authors, who have followed the glori- ous tracks of the great Entomologists of the past century. Eminent amone such exceptional works containing more than descriptive details, stands the remarkable report of Dr. Harris upon the In- sects of Massachusetts injurious to Vegetation, in which the author has given most valuable inform- ation upon the metamorphoses of insects living in this State. Also, Professor Audouir has given many beautifully illustrated facts about the insects injurious to grape vines. Ratzenburg has made similar investigations on insects injurious to the forest trees in Germany. To these works we shall have constantly to refer when studying the meta- morphoses of articulated animals. The larvae differ from each other, not only in form but also in structure, and in the successive changes which they undergo. There are larvae which arise from the egg almost under the same form as the perfect insect, and in their metamor- phoses undergo only slight changes of form; per- haps changing the length of their legs, or modify- ing the apparent number of rings which they had when coming out of the egg. There are others which are born widely different from the perfect insect, which will remain in that form for a certain time, and then change into an Animal entirely dif- ferent in its outline—to remain in that condition again for a longer or a shorter period, and then to undergo the last transformation. Insects which undergo such complete changes in form, are called insects with perfect metamorphoses. Those into which changes are introduced gradually, and in which the differences in various periods of life are not so great, are called insects with imperfect met- amorphoses, or half metamorphoses. We have insects in which the young are born under nearly the same form as the perfect insect. I would men- tion the Grasshoppers, for instance, in which the young have the same forms except the wings, which are wanting. The greatest differences are noticed among Butterflies (Plate H, fig. C), where [Plate II—Caterpillar Pupa & Butterfly.] the Caterpillar is first seen (Fig. A), next the Pupa , (Fig. B),and lastly the perfect animal (Fig. C); also I in the Beetles (Plate XI, Fig. D), where the form ^ represented by figure E, is first seen; next the Pupa (Fig. F), and then the perfect condition (fig. D). Fig. A. represents another Beetle in which the larva (Fig B) is similar to the Caterpil- lars. In most insects, the larvae, when colorless, are called Maggots,or Worms In the Ephemera (Plate X, fig. L), we have the same form of the body as is seen in the perfect insect; but on the sides of the larva there are aquatic respiratory organs gills, (Fig, L,) which do no longer exist in the perfect insect (Fig. J). Such cases indicate the extensive differences of structure which may exist among larvae of the same class. [Plate XI—Beetles with their Larvae and Puvje 1 Some (Plate X; nave aquatic oreatning organs, and others aerial ones—a difference which in oth- er departments of the animal kingdom is consider- ed sufficient to divide some of them into different classes. Fishes and Reptiles are not left in the same classes, because the respiration of the one takes place by gills, and in the others, by lungs.— You will notice in this figure, (Plate X, fig L) and in Plate XI, fig A, considerable differences : in the one there are gills, and in the other lung-like or- gans for the same function. In others we see still different combinations. In the Phryganea, for instance, (Plate X, fig. H) there are legs only upon the anterior rings, and there are stiff hairs upon the other rings; whilst in the Caterpillar (Plate II, fig. A) there are legs upon the anterior part of the body; others on the mid- dle joints ; and still others, behind. The larva of the Horsefly (Plate IX. fig F) ha8 no legs at all, only stiff hairs. In the Mosquito (Plate IX, fig. C) the larva is aquatic, provided with gilis. The pupa (Fig. B) assumes another form, but remains aquat- ic, and finally, the animal appears with legs in a very different form (Fig A) and with a pair of long wings and various appendages in addition. Now, it is important—I'insist upon this point— * not only to trace the charges which the larva* ,,„. LECTURES ON EMBRYOLOGY. 53 dergo in their metamorphoses, but also to investi- gate the changes in their structure, which are brought about during their metamorphoses; and happily we have upon these points most admira- ble investigations by Dr. Herold, though upon only one species, the white Butterfly which feeds apon the cabbage. It is remarkable, however, how few investigations have been made upon these animals at large, when we take all points of view into consideration; and we find ourselves reduced, for illustration to one single well studied exam- ple. Prof. Herold in his admirable work begins, unfortunately the investigation only with the full grown Caterpillar, which he goes on comparing with the pupa, and then with the perfect insect. Now with reference to these differences between the larvae—before I allude to peculiar differences of structure—let me make another general remark. There are groups of insects in which considerable differences occur among the larvae, even in their structure, when the perfect insects constitute nat- ural families, and are identical in structure. Again, there are others, the Butterflies for instance, in which the larvae agree as perfectly as the full grown insects, having alia distinct head (Plate II, fig. A), with powerful jaws,and a slight indication of eyes. Then, we find upon the three anterior rings there are three pairs of legs provided with horny claws, next two rings without legs at all, then, rings with feet of an entire different structure, resembling suckers, then two rings without legs, and a pair of legs upon the last ring. And this arrangement of parts is uniform through all Butterflies. It occurs in the Diurnal as well as in the Sphinx and Noc- turnal Moth. The larva of Butterfly is never an aquatic animal, but is always an air-breathing crea- ture,but there are many aerial insects whose larvae are entirely aquatic. Another difference is, that these insects in their lower condition have powerful jaws, by which they chew their food, moving their jaws from right to left and from left to right, on the two sides; while the perfect animal is very different in having no longer jaws to chew the food, but suckers to take food from the nectar of flowers. And the change in the mode of living is so great, that the Caterpil- ler will consume ten times his own weight of food in a given time, while the perfect animal will not consume more than one tenth of his weight during all the remainder of hfs life, as a perfect insect. This fact has great importance in connection with one question about which Naturalists have had much discussion, viz: whether the insects which chew their food should be considered as higher than those which suck their food by suck- ers. The Insects provided with powerful jaws— the Beetles, the Wasps, the Humbiebees, Dragon- flies—all these insects, which have powerful jaws, are generally considered higher in their structure, because so many of them are carnivorous, and stand in our systems as at the head of insects; whilst the sucking insects are placed in a lower range. That the former are placed higher, arises from no other reason, I think, than the fact that there are so many of them which live upon ani- mal food, or which are properly carnivorous : and as we are accustomed from our intimate acquaint- ance with mammifera to consider Carnivora higher than Herbivora, we are naturally misled to con- sider all carnivorous animals, for the simple reas- on, that they are carnivorous, as higher than the herbivorous ones. But such impressions can have no value in the estimation of the characters of an- imals of another department. The larvae of many sucking insects have equally powerful jaws as the carnivorous, which are made into another appara- tus of an entirely different structure, introduced in the last transformation of the insect [Plate V—Articulata—Trilobite.] My impression is, therefore, that on this account we should rather incline for an inverse view of the subject, and an inverse arrangement of the insects, and consider the sucking insects as higher than the chewing Insects. And I would place the But- terflies highest, for the reason that they undergo such extensive metamorphoses—passing through so many changes in which the structure grows successively more perfect. That they should be placed highest amongst the sucking insects will be obvious, when we consider that they are aerial worms from the beginning—while other insects, with the sucking apparatus, as Flies and Mosqui- toes, constitute a family in which there are many aquatic worms, and we know from other depart- ments, that aquatic animals provided with gill- like apparatus are universally, lower in structure than those which breathe air. But such an uni- formity in larvae as we have among Caterpillars is not noticed in other insects. You can of course compare the larva of Dermestes (Plate XL, fig. B ) with a Caterpillar, (Plate II., fig. A.) But. of the external appearances, the appendages of the skin agree; the arrangement of the feet will be found different. The aquatic insects have their larvae still more different, being provided with gills, so that the ex- ternal form in its earlier condition, is far from uni- form in the families which reckon aquatic types. Among the hymenopterous insects, Bees, Wasps, &c-, we have some in which the larva assume the form of Maggots and Worms, and others in which the larvae assume the form of the higher insects. For instance, in Tenthredo, the larva assumes the form of a Caterpillar. (Plate II., fig. A) But in- stead of having only four pairs of suctorial feet, thev have seven. And this is at once an indica- 54 PROF. AGASSIS S tion that they do not belong to the family of Lep- idoptera. I see the time will not allow me to go through the whole of this extensive subject; so that I shall 'call your attention again in my next lecture to the transformation of structure which takes place in these animals. Let me only make one remark more with reference to the relative position of the various families of the numerous order of insects, and to the relative value of their distinguishing character. Why should we be led to arrange the insects and articulated animals in a natural order, by other considerations than those derived from their own mode of growth ? For, if we find that in insects the earliest period of life is that of the car- nivorous animals, let that be the lower condition for articulated animals. And if we see that they successively undergo changes, in which, growing to our eyes to more perfect animals, they finally assume the structure of sucking insects, then let us consider the condition of sucking insects the higher condition. And let ns no longer transfer our impressions from one department into the other. The same difficulties occur really in all other classes. Because the Carnivora among Mammalia, come so near to the Monkey, and thus approach to the affinity which raises the Monkey next in rank to man, it is no legitimate consequence, that the Birds of Prey should be the highest. Nor does it prove that the carnivorous fishes should rank high- er than the others: and still less, does it follow, that the chewing insect should take the highest rank, especially when we see that the chewing condition is the lowest embryonic condition of their life. And let us, in future, arrange insects according to the rule of insects, and not according to the struc- ture of other animals. LECTURE VII. Before entering upon the proper subject of this evening's lecture, I have to mention a few facts which I have ascertained upon the growth of some Pblypi (or rather Medusae, if Tubulariae have to be considered as Medusae) which I consider so highly valuable as to deserve really to call our attention for a few moments. I have received from Mr. Hawks, of the Navy Yard in Charlestown, a bunch of Polypi, taken from the bottom of a ship which has been lying for three months and a half in the harbor. When she was launched, on the 14th of September last, she had, of course, none upon her. She had been lying in the water from the 14th of September to the 28th of December, when she was taken into the dry dock. During this time, the bottom of this ship has been covered with the most astonishing, the most luxuriant growth of Polypi which can be imagined. Thousands and thousands of Polypi stems, as long as five, or six, or seven inches, forming the most beautiful flower garden, upon the bottom of the vessel. And not only have all these Polypi grown to this size, but they have branches, and these branches—these secondary branches—have given rise to branches of a third order, in this short time. Now, the question is,how can these innumerable stems have grown upon this vessel ? They could not have been attached to it accidentally, as Tubulariae, in their ordinary growth, are always attached, and when freed, fall to the bottom, without having the means to move about. The uniformity of their growth, shows that they have grown upon the vessel from a uniform starting point, not from a certain number of stems which had accidentally become attached to the vessel; all of which must be supposed in the same condition, in the same state of growth, when they became attached, and that they have grown upon this vessel naturally, uniformly, up to the present day, or rather up to last week. But to be attached there, in such a manner, not accidental- ly, thev must have been free *, and it is just a point to which I alluded in a former lecture, whether Tubulariae had or not, a free generation, alter- nating with their fixed growth. A free generation among them is not known; yet I inferred from some data, that the affinity of Tubulariae with Me- dusae was very close, and I ventured even to predict that some one of the small free Medusae of Boston harbor might be their free form—that a free gen- eration might be found. Now, the circumstances above stated, show that there must be a free generation of Tubularise,which, by the 14th of September, or some time later, were swimming in Boston harbor in countless num- bers, and attached themselves to the keel of that vessel, and grew there to form these innumerable stems. Whether this growth is immediately de- rived from the germs, which are produced in the bunches, which are known to exist in Tubularia, or whether it is only another generation, derived LECTURES ON EMBRYOLOGY. 55 from that free one, is still a point which only di- rect investigation can ascertain. I incline to sup- pose, that the Medusa-like germs which are devel- oped from the bunches of eggs, hanging below the outer tentacles, are the intermediate, free genera- tion which grows to lay moveable eggs, similar to those of Gampanularia; and that these eggs, and not the soft free buds, grow into Tubulariae. How- ever, so much, at least, can already be inferred with precision: that Tubulariae must have seme free generation,—a generation which is about to attach itself in the latter part of September, and to produce a luxuriant growth of common branching Tubulariae. Now, how rapid this growth mHst have been, and how rapidly the branches must have succeeded., an illustration of the details will show. Each single, isolated stem, from fife to seven inches long, ter- minates with a crown, having its tentacles and bunches of eggs, like the most perfect Tubularia I have seen. The terminating Polyp has bunches of eggs, and all these eggs have already their yolk, with its envelope—their germinative vesicle, with its germinative dot. The lateral branches, per- haps five or six, in various stems, growing from different parts of the stem (bat the lower always, in every case, being longer than the upper ones,) were terminated also with regular crowns; but those smaller and simpler individuals, the number of their tentacles being fewer, were found to be without any eggs. They had net grown to the formation, to the development of organs of repro- . duction. The tertiary branches, sometimes as many as five or six upon one of the secondary, were found to terminate also with a small Polyp; but like the secondary, to be without eggs. Hun- dreds of these branches, compared together, show- ed no difference. They were so alike as to indi- cate, distinctly, that they were the growth of one epoch i—that they had been attached to the vessel at one time, and had grown under identical cir- cumstances. That stems already formed, could not be attached to the vessel, is shown by the cir- cumstance that the loose branches sink to the bot- tom, and have no means of transportation from one place to another. Thus.the being which was fixed, must have been a free animal. You remem- ber, perhaps, what I have said in a former lecture upon the embryonic growth of Tubulariae. I showed the formation in the bunches of eggs of little Medusa-like beings, with four or more arms, —four prominent ones, and others alternating with them, less developed, which became free, but whose final development had not been observed. I now suspect that these Medusa-like buds would grow into Medusa-like animals, and that these Medusa-like animals would lay eggs, and that these eggs, like those of Campanulariae, being first free, would then become attached—grow to a disk- iike surface, rise from the centre to a stem-like growth, and then pass through the same meta- enorphoses which have been observed in the Cam- panularise. At all events, here is one fact in the history of this animal ascertained, which was un- known before—the fact of its rapid growth, of its rapid branching; and of the existence of a free generation, though not ascertained by investiga- tion, so strongly indicated by circumstantial evi- dence as to be almost a positive fact, in the opin- ion of one who has been accustomed to compare these phenomena arid to refer them to a common type. In my last lecture, the first upon articulated an- imals, I began by illustrating, in a general man- ner, the character of the great and numerous type of Articulata; how they are subdivided into three classes—the Worms, Insects and Crustacea—or in the order which I would prefer, Worms, Crus- tacea and Insects; then further, I alluded to the pe- culiar characteristics of insects, to their extensive metamorphoses; and then more fully illustrated the embryonic growth of these animals, as as- certained by the investigations of Professor Kolli- ker; and finally investigated the different meta- morphoses in different families of Insects. We now proceed to the investigation of the changes of structure which these insects undergo during their metamorphoses. We have examined the general changes of form which these animals undergo in various families. We have now to ex- amine the changes in the internal structure, which take place in the larvae of Insects, till they acquire their perfect development. And in tracing these changes, we shall acquire an invaluable key to ap- preciate the relative value of the differences which exist between all insects—between articulated ani- mals at large. If it is true that Insects are the highest among articulated animals, even if they should occupy a second rank, a thorough acquaintance with all the changes of structure which they undergo during growth, mast give us a key to appreciate the real value of these differences, their relative order of succession in a scale—in a gradation of structural differences. The value of these comparisons must be so ob- vious, that I need not apologize for dwelling more extensively upon these topics than I would other- wise. I repeat it—that the facts which we are now about to examine will furnish (if there Is one) the key for estimating the value of characters in one of the greatest types of the animal kingdom. In Plate XII are diagrams representing the ner- vous system of a White Butterfly.(which is exceed- ingly common in Europe) living upon cabbage, in its various stages of growth, as figured by Herold,'in his remarkable work upon the metamor- phoses of that animal. In Plate XIII are diagrams representing the changes which take place in the digestive apparatus of the same animal; and here (in Plate XIV, figs. A and B) are represented lon- gitudinal sections of a Moth (Fig. A) and its Cater- pillar form (Fig. B) from Prof. Newport's research- es, to show the different systems of organs in their 56 PROF. AGASSIZS relative position within, and also the changes which they undergo during their growth, as well as in their proportional development. To these diagrams I shall mostly refer daring this illustra- tion. But in such a comparison of structural dif- ferences, the external arrangement of parts is as mportant as the internal differences. We have examined the forms of the various sta- ges of growth in Inseets. We have not examined the differences in the arrangement of the external parts. Let as begin the comparison with these. [See Plate IX, Lectare 61 In the various Caterpillars or Maggots—in the various larvae of Insects which you see figured in Plates IX, X and XI, and Plate II of the first lec- ture, there is one form which is characteristic in all—which occurs universally in all. It is the greater uniformity of rmgs when compared to each other. The rings of the anterior part of the body, (Plate X, fig. H, or Plate XI, fig. B) [See Plate X. Lecture 6.) though here provided with legs, resemble the rings in the middle portion of the body; however, they resemble these more closely than the anterior rings resemble the posterior ones ; bat as a whole, considered in its general arrangement, the various rings of the larvae are more uniform than in the perfect insect, which arises from them \ and they are naturally more uniform, hot they are not grouped together in any particular way. There are no differences in the rings, indicating more circumscribed parts of the body. Scarcely is the head more defined from the other rings by its co- lor. But, between the so-called ehest of Insects and the abdominal region, there is no separation (see Plates IX, X and XI) as we notice it in the perfect insect. There is always in the perfect insect, between the head and the chest, and the posterior part of the body, a strong division, as we see in these fig- ures, (Plate XI, figs. D and A) where the head is more distinct; a certain number of rings consti- tute another region behind the head, the so-called thorax, or chest; and behind this, there is a third one—the abdomen. Now, such a division of rings into distinct divisions—into a head, thorax and abdomgn—is not yet introduced into the condition of the larva, though it is indicated by the appen- dages ; though not universally, but very general- ly, there are among the anterior rings some which have appendages more developed than the others, which will correspond with the rings which form the chest, and then the other rings behind will correspond with the rings which form or constitute the abdomen. But now, compare the proportional size between those rings in a perfect insect—Grasshopper tor Instance as in Plate XV. Here (Fig. A) is the head. This middle region, here separated into its constituent rings, (Figs. B, C,D>will correspond with the ehest \ and here, [Plate XV—GBA88Hor?a«.Tj_ posteriorly, a portion ot the body, scarcely larger than the head and thorax together, though com- posed of twice as many rings, corresponding to the abdomen. In the imperfect larva (.Plate XI, fig. E) we have precisely the reversed proportions in the size of the rings of the different regions of the body, or what will finally constitute these differ- ent regions. The posterior rings in this case are reduced considerably in the perfeet condition, bul the rings giving rise to the thorax are enlarged, and closely united in fewer joints, so that there is a real reduction of rings, and a real redaction of the moveable parts, inasmuch as the three rings of the ehest, which in the earKer stages are equally moveable npon each other, now are united togeth- er, and form only one mass. The reduction, there- fore, of the number of rings or their closer com- bination, or the reduction in size of the posterior ones, with a proportional increase of the anterior ones, when they acquire a higher development, are stages of growth which indicate a progress—a really progressive development. From these first superficial investigations, we learn one important fact in Entomology—that elon- gated species, in any given type, consisting of well divided, uniformly moveable rings, must be con- sidered as lower than those in which the rings combine or nnite together, and divide into distinct regions. So that the Caterpillars give us the firs» hint towards a classification, namely, that Insects, or Articulata at large, stand higher or lower, inas- much as the rings are more or less numerous or reduced, uniformly moveable or combined, uncon- nected, or united into distinct regions. ISee Plate IV, Lecture 6 ) LECTURES ON EMBRYOLOGY. 57 And if we test with this first result the proposed modifications in the general classification of Ar- ticulata, we will find that on this ground Worms (Plate IV) will stand lowest, Crustacea (Plate VI) come next, and Insects highest. [See Plate VI, Lecture 6 ] Let us now examine the changes which take place in the nervous system of the Caterpillar when full grown, (the changes during the growth of the Caterpillar itself have not yet been investi- gated) till it is transformed into a perfect Butter- fly. We have at first, a nervous system, consisting of a series of equally developed and almost equally distinct swellings (Plate XII, fig. A)—in the head two large ones ; next, one small oce; at about an equal distance, a second; a third, nearly equally distant: a fourth, somewhat more distant; a fifth, sixth, seventh, eighth, ninth, tenth, eleventh, al- most uniformly equally distant; and then a twelfth, which is nearer the eleventh, making, with the head, thirteen. Now, precisely the same number of nervous swellings which we observe, consti- tute the number of rings existing in the Caterpil- lar. Uniformly throughout the family of Lepidoptera, that is to say, among Butterflies and Moths, the body consists of thirteen successive rings ; and in the lowest condition of these animals—in their caterpillar state—the nervous system has as many nervous swellings,—one for each ring, almost equally distant from each other, and sending off threads to the parts around in each ring. The general structure and position of the nervous sys- tem is as follows:—The swellings are throughout united by double threads, which towards the poste- rior part of the body come so near together as to seem a continuous, thick cord; but properly speak- ing, they consist uniformly of double threads. And in the position of these threads, there are some im- portant points. The anterior ones are above the alimentary canal; the others are below; so that the thread which unites the anterior ones with the second,constitute a sort of collar around the alimen- tary tube (PI. XIV). But all the swellings are united by double threads, even where the threads come near together and seem to be one continuous cord. I insist upon this point, because it shows the uni- formity of structure of the nervous system in all articulated animals, and illustrates it, even in the structure of the nervous system which has recent- ly been discovered in Intestinal Worms. When discovered, it was supposed that Intestinal Worms had a nervous system so different from Articulata as not to belong to that group. The nervous sys- tem in Worms forms a sort of collar, with swellings around the anterior part of the alimentary canal, from which arise a double row of swellings, con- nected by simple threads, extending backwards.— This arrangement? is indeed not very different from that of the higher Articulata: let only swellings, with their double threads, be disconnected, and we have the arrangement of Worms; and let the two chains of Worms be united in one, and we have the arrangement of Insects. As soon as the Caterpillar undergoes the first change towards forming the Pupa—towards be- coming immoveable, before it casts its skin for the last time—we see (Plate XII, fig. B) that the third and fourth swellings are brought nearer together; and also the first and second are brought nearer together; the others remaining in the same relative position and in the same proportional distances apart. But as soon as, for the last time, the Caterpillar has lost its skin and assumed that peculiar form of Pupa in which it is motionless, then the nervous system in its longitudinal extension assumes this winding form [Plate XII, fig. C.J It brings the swellings nearer together, the first of them being at this time entirely united with those at the head- [Plate XIT—Nerves of Butterflies.1 In the following stage, (Fig. D) the 2d, 3d, 4th and 5th swellings are brought nearer the head,whils the 6th, and 7th, disappear entirely during the pu- pa state, and with them disappear also the lateral threads which arose from them in an earlier con- dition. The second, third, fourth and fifth swellings remain now for some time at the same distance, but are gradually combined in one single and more connected mass. The sixth and seventh, disap- pear. The eighth, ninth, tenth and eleventh re- main at equal distances. And if we compare this condition with the perfect insect, we can see that these few anterior swellings, though arising from five distinct ganglia, will send the nerves to the parts answering to the chest. A region behind, with the long medial thread without lateral nerves, is the region where the separation between the chest and abdomen will take place. Before the Pupa passes into the state of the perfect insect, the approach of the swellings number two, three, four and five is still increased. So that there are now only three regions of distribution of the ner- vous centres: the head with one large mass; next, the chest with separted, though approximated 58 PROF. AGASSIZ'S swellings; next, a great spacewithout lateral nerves; and then, a space with swellings at equal distances, corresponding to the abdomen. Remember now the arrangement of rings and legs in the Caterpil- lar and in the Grasshopper, [Plate XV], You will see that the arrangement of the external parts agrees with that of the nervous system. The head consists of one undivided mass [Plate XV. flg. A ] There are three pairs of horny claws in the Caterpillars (Plates IX, X, and XI,) and three rings to the chest in the Insects proper, (Plate XV, figs. B, C, D) receiving nerves from the concentrated swellings of the anterior part of the body. Then, there is a region from which no nerves are deriv- ed ; and a region from which four pair of sucker- like legs are produced, answering to the region in which these four swellings remain equally distant; and then another region, of two rings without; and another, last, with suctorial legs, which corres- ponds to the large terminal nervous swelling. It is a question which it is not possible to solve now, and which it will be very difficult to solve, if it can be solved at all, whether the larger terminal swelling of nervous matter consisted originally of one nervous mass; and whether the anterior ce- phalic ganglion consisted also, primitively, of one nervous mass. That it consists of two now, is shown here, [Plate XI. fig. A] by the entire disap- pearance of the first small ganglion. But there may be other changes in the structure of the ner- vous system, taking place previous to the full growth of the Caterpillar. And this remains for the present undecided. But, so much is shown as to prove that the nervous system is equally dis- tributed in the solid rings, and they will gradually combine in such a manner as to present arrange- ments answering to the changes which take place in the external form. There is one mass more, properly belonging tc the head, another mass more concentrated, belonging to the chest, and another mass remaining stationary and belonging to the abdomen. We now can, with these facts, arrive at another general conclusion, viz : that wherever among ar- ticulated animals, among Insects, we find the ner- vous system constituted of equally distributed ner- vous swellings, such animals are lower than those in which several swellings unite together to form few masses. Now, in this respect, what do we ob- serve in the different classes compared together 1 I now no longer compare the same animal in its different stages of growth, but different classes of Articulata with each other. What do we observe in comparing Insects with Worms,and Worms with Crustacea ? All worms have equal rings and very numerous joints ; and joints which are never com- bined so as to form regions distinct from each other. There is never a distinct thorax or abdo- men in any Worm. So that, from what we have learned, we know that the lower position assigned, for many and all sorts of good reasons, to worms, is the proper position which they must preserve; and where a nervous system has been observed among them, it agrees with the condition of that system in Caterpillars, rather than with that of the later metamorphoses. The question remains be- tween Crustacea and Insects. What is the condi- tion of the nervous system in Crustacea1? The nervous system occurs in various conditions there. In the lower Crustacea,tbe swellings being scatter- ed all along the body, one to each ring—a condi- tion which we observe in the earlier stages of growth in the Caterpillar. Next, we have other Crustacea in which the nervous swellings contract and combine together, nearer and nearer. But in them, strange to say, there is only one point of concentration. And then there are Crustacea, as the Crabs, in which the nervous system is con- tracted into one single, central mass. And the question is, what shall we consider superior1?—an arrangement which gives rise to several distinct centres, and corresponds to distinct regions of the body, (as in Insects, Plate XII., fig. F. and Plate XIV, fig. A) a head,with a central nervous swelling of a peculiar kind ; a chest, with a nervous mass of a peculiar kind, sending its thread to the legs of that region; and another posterior combination of nervous swellings, corresponding to the other re- gion, called abdomen, and sending nerves to its part? It seems to me that we cannot remain doubtful. We cannot fully derive this conclusion from direct investigations, as we have not, in any instance, a case to settle it by direct comparison; but we may say,that in Crustacea we have concentrated unifor- mity ; while in Insects in their perfect condition, we have concentrated diversity. And, if we are al- lowed to compare the one with the other, I would incline to the opinion that concentrated diversity, with prevailing influences over peculiar functions of the life of the different centres, is a condition of structure which stands higher than concentrated uniformity,in which we have only one centre. We have all the primitive diversity reduced to one centre, which does not acquire any distinct influ- ence upon different parts. The alimentary canal undergoes corresponding metamorphoses. Here is the straight tube (Plate XIH, fig. A) of the digestive canal of a Caterpillar. It is very wide in comparison to its length, and ca- pable of digesting an immense mass of food, com- paratively to tho size of the animal. In its earlier condition, it is provided with an apparatus which disappears afterward. There are considerable sali- vary glands in the anterior portion of the alimen- tary canal, which disappear in the pupa Btate and do not exist in the perfect insect. These figures (Plate XIII) must impress you as very singular.— No animal has more curious organs than this In- sect. The liver, or hepatic glands, and the salivary glands are massive organs in other animals. Here, they are slender tubes, and form little winding branches on the Bides of the alimentary tube. In- eed, all glandular organs in Insects have such a LECTURES ON EMBRYOLOGY. 59 [Plate XIII—Alimentary Canal or Butter- flies ] (Plate XIII., figrue C.) The animal has now ceased to take food, and the salivary glands dis- appear entirely. (Plate XIII., fig. D ) Next, the colon grows more slender, to be transformed into a narrow cylindric tube. When the Pupa is ready to be transformed (Plate XIII., fig. E,) into a But- terfly, there is a new pouch formed between the oesophagus and stomach, a pouch which secretes the honey. It is a sack, to produce the sweet fluids which so many insects are capable of secreting, or at least of preparing. This pouch (Plate XIII., fig. E,) has grown to a somewhat large size, and the posterior part of the alimentary canal has been elongated very considerably, in proportion as the middle part or the stomach proper has beeu re- duced. And finally, in the Butterfly, it is fully developed, but we see no longer any salivary glands. (Plate XIII., fig. F.) The posterior part of the alimentary canal is now long and slender, and the hepatic duct of the liver nearly as large and as complicated as in the beginning. Here again, we see that in proportion as the ali- mentary tube is a uniform tube—or in proportion as there are cavities of different diameters devel- oping along its longitudinal diameter—we have another scale to determine the relative rank of an- imals in which this organization is observed.— [Plate XIV.—Longitudinal Section of Sphinx Ligustri.] This is, perhaps, better seen in another diagram of a Moth, where we see the oesophagus passing through the anterior nervous ring, and extending in the perfect insect PI. XIV. (Fig. A) through the chest, where the wings are cut off and the legs also. The large thorax answers to that part of the Caterpillar (Figure B,) where the horny legs are seen, and the ganglionic portion of the nervous system is seen all along below the alimen- tary canal. And in the Caterpillar you see how intimately and uniformly the nervous swellings follow each other, (Plate XIV, fig. B) and how the alimentary canal is a uniform tube, whilst in the perfect insect, alimentary canal and nervous sys- tem have undergone remarkable concentrations (Fig. A). Another apparatus is very simple among Insects. It is one of those functions which is not so high- ly developed as in other Articulata, but which, general arrangement—they are all tubular, thread- like, and very long. The next glandular apparatus here (Plate XIII. fig. A.,) is the gland seen on each side, behind the salivary tubes, the silk glands, which are much larger in the Caterpillar than in the perfect in- sect. These silk glands still exist in the perfect insect, but they are much larger in the Caterpillar than in the Pupa, and again larger in the Pupa than in the perfect insect. You are aware that the Caterpillar draws its silk from its mouth, winds it regularly around its body, to protect it during its second stage of metamorphosis. The third gland- ular apparatus, a kind of liver, consisting of three pairs of hepatic tubes, emptying in the posterior part of the wide tube of the Caterpillar, but about its middle in the perfect insect. This condition of the glands, which we find among all the Insects, is far from the structure of those massive glandular organs which occur in other animals. The lower portion of the alimentary canal is scarcely at all contracted, in the Caterpillar, as you will observe in this figure (Plate XIII., fig. A ) Before entering the pupa state (Plate XIIL, fig. B,) at a period when the insect is more perfect, the oesophagus has become narrower and longer; and the colon has also become more elongated and narrower, and in the pupa state you see how the digestive tubes appear. 60 PROF. AGASSIZ'S nevertheless, exists. There is a circulation in In- sects which is only more generally overlooked.— The heart is a more elongated tube than in Crus- tacea, but it exists in all insects. It exists more developed in their larval condition, which shows that having a large heart in articulated animals, is not characteristic of a higher structure; and how a great bulk of blood can be concentrated upon one point in Articulata, without assigning them a character of great eminence, is distincly shown, when we consider that in Worms, which undoubt- edly stand below the other two classes, there are as many as six. eight or more hearts, and in which the bulk of the blood is proportionally much great- er than in Crustacea or in Insects ; so that, the im- portance ascribed to the circulation of Crustacea, when this class was placed above Insects, I think vanishes before the consideration of the value of these characteristics, as noticed throughout the metamorphoses of Insects. A few words upon the subject of mastication and upon the chewing orders, will further show that Insects have to stand higher than other articu- lated animals. The chewing apparatus in Insects is a very complicated apparatus, so complicated that it is scarcely possible to give a correct idea of the arrangement of these parts, unless a person has become familiar with the objects themselves. I must, however, attempt to convey some idea of this apparatus. On the two sides of the head in those insects which are generally considered the highest, there are two large moveable pieces, mov- ing from right to left on the right side, and from left to right on the left side, in opposite directions horizontally. These parts are called mandibles.— Below thesa, is another pair of similar organs, moving also horizontally, which are often ser- rated, and to which are frequently added articu- lated appendages; these are called the maxillae.— These constitute two pairs of strong forcep-like jaws, very different, it seems, from any part in the whole insect. In the diagram here, Jaws of Insects (Plate XVI. figs. A, B) your see the whole apparatus, first from a Beetle and a Grasshopper, (fig. C). Seen from above (fig. A) there is a kind of lip in sight, cov- ering the mandibles, and below, are the maxillae; and below (fig. B) there is another kind of lip.keep- ing these in their respective positions. To the lower lip are also frequently appended articulated tentacles—the palpi. Fig.C. represents the maxillae of a Grasshopper seen in profile. Now, each of these parts being taken asunder, we will have a strong mandible above; and some- what below and inward, the maxillae; and farther below, we have the lower lip. So that, between two horizontal continuous plates, called lips, there are moving forceps, the upper, called mandibles, and the lower maxillae. Then we have maxillary pal- pi. And to the lower lip there is another pair of palpi attached—the labial palpi. This is the structure of the jaws in all chewing [Plate XVI—Jaws or Insects] insects. The Caterpillars have also such maxillae as the perfect chewing insects, though not so com- plicated, to be sure, as in the most perfect Beetles, but nevertheless constructed in the same way,with a horny, powerful jaw, by which they chew the large quantity of food which they devour. Now this condition is changed in the Caterpillar during the pupa condition, when we have no longer such enlarged jaws; but a long sucker, [Plate XVI fig. D] consisting, however, of the same parts as in the chewing insects, only those part6 which were mov- ing horizontally have become elongated, and with their margin have united, and instead of now mov- ing in that way, remain closed together, and form, a tube, a real sucker, through which, by the assist- ance of the tongue, they actually pump liquid food into their stomachs. (The Professor here repre- sented, by means of his fingers, the jaws of the chewing insect, and the manner in which, by uni- ting, they can be transformed into a sucker.) Let the tube now be contractile and retractile.be- tween the upper and lower lip, and you have pow- erful jaws transformed into a narrow tube. It is a transformation which takes place with the other successive and progressive changes, so that we are entitled to consider such changes as also a pro- gress, if I am not mistaken ; and to consider the condition of the insect in which he chews food, as the lower one, as it is the condition of the Larva; and the condition in which he sucks, to be the higher condition of the insect. And therefore, in principles derived from the study of Insects, and not from the study of other animals, judging of Insects by notions gained from that class, we shall consider those which suck their food, in which the jaws are elongated, those which pass through vari- ous metamorphoses, higher than those in which the jaws are placed horizontally—sharp cutting jaws,which devour large quantities of food. But this LECTURES ON EMBRYOLOGY. 61 condition of jaws, I say, is of higher structure than Chat which is observed in Crustacea; and affords an additional evidence than Insects should stand above Crustacea. To show this to be the case, let me first answer a question. What are these jaws in Insects ? By most difficult and extensive com- parison, it has been ascertained that the jaws are simply modified legs, and that there are all possi- ble transitions to be observed in the various fami- lies, between their ordinary legs and that peculiar kind of moving appendages which perform, the function of jaws, but which are so exceedingly dif- ferent, owing to the great eminence in form to which they arrive Now in Crustacea, the changes which take place between the appendages functioning as legs, and those functioning as jaws, are so slight as scarcely to present any difficulty in ascertaining their com- mon nature i the differences are much less plain in Insects, with their different sorts of jaws. You scarcely can find the combining thread, showing that in Insects there is one, and an uniform modi- fication of appendages in legs and jaws. But com- pare, on the other hand,various appendages of the Crustacea, and it strikes us at once that they are the same thing, slightly modified. Before I illustrate this point, let me remark, that on looking at this diagram, there is scarcely any one who would suspect that these figures represent any thing more than the various claws which are observed on the side of the lobster. And so it is ; but nevertheless, some are jaws, others claws, oth- ers fins; the jaws being somewhat modified legs; so that those parts are only a little diversified among each other. We have something left of uniformity; while we rise in Insects to the greatest possible di- versity, and even a diversity which presents an analogy with the character of concentration, ob- served in the various arrangements of their nerv- LE condition of jaws, I say, is of higher structure than Chat which is observed in Crustacea; and affords an additional evidence than Insects should stand above Crustacea. To show this to be the case, let me first answer a question. What are these jaws in Insects ? By most difficult and extensive com- parison, it has been ascertained that the jaws are simply modified legs, and that there are all possi- ble transitions to be observed in the various fami- lies, between their ordinary legs and that peculiar kind of moving appendages which perform, the function of jaws, but which are so exceedingly dif- ferent, owing to the great eminence in form to which they arrive Now in Crustacea, the changes which take place between the appendages functioning as legs, and those functioning as jaws, are so slight as scarcely to present any difficulty in ascertaining their com- mon nature i the differences are much less plain in Insects, with their different sorts of jaws. You scarcely can find the combining thread, showing that in Insects there is one, and an uniform modi- fication of appendages in legs and jaws. But com- pare, on the other hand,various appendages of the Crustacea, and it strikes us at once that they are the same thing, slightly modified. Before I illustrate this point, let me remark, that on looking at this diagram, there is scarcely any one who would suspect that these figures represent any thing more than the various claws which are observed on the side of the lobster. And so it is ; but nevertheless, some are jaws, others claws, oth- ers fins; the jaws being somewhat modified legs; so that those parts are only a little diversified among each other. We have something left of uniformity; while we rise in Insects to the greatest possible di- versity, and even a diversity which presents an analogy with the character of concentration, ob- served in the various arrangements of their nerv- LE I hoped to introduce another subject, which is connected with the history and metamorphoses of Insects; but my time is so short that I scarcely dare to mention it, only as connected with these investigations. I mean, the singular peculiarities •f many insects who live in large communities.con- sisting of individuals of different kinds, combined in various numeric proportions, among which there are not only Male and Female, but also an- other kind of individuals, differing from them, called Neuters. In those communities, individ- uals live in various combinations; there being, for o ous system, compared with that of Crustacea. So that here we have another, and perhaps one of the best, indications, that Insects stand higher than Crustacea, notwithstanding we have the anatomi- cal evidence to the contrary, which has been relied on. Now if upon these data we should attempt a classification of the class of Insects, let me in a very few words make it clear. Insects have generally been divided into chew- ing and sacking Insects, and then into other fami- lies. Spiders have been separated as a class, and also all Apterous Insects. Now, the Millipedes rank lowest, as among In- sects they represent the form of Caterpillars or Worms. Next the Spiders, in which the concen- tration takes place, in which the head and thorax are distinct from the abdomen, but in which head and thorax are not separate, as in other insects indicating some analogy to Crustacea. Then, we would have those in which head, chest and abdo- men are separated; but among them, place those in which there are chewing jaws lower; and high- est the sacking insects. And curious it is, among those which chew their food, that we have the less perfect metamorphoses and many which are aquatic in their larval condition; also among them the forms are less perfect, inasmuch as in Neuropterous insects the parts of the thorax are only partially united, and the number of joints remain greater even in the perfect insect; while in the sucking insect, the parts of the thorax unites in one mass, distinct from the head. In the Butter- fly, we have, the evidence in the earliest lar- val condition, that the Worm is an aerial animal, rising above the other insects. And with these data I think I have shown that I am not wrong in considering the Insects as highest, if we judge upon entomological grounds and not upon other evidence. « IB VIII. instance,in a bee hive, one Female, a Queen, as she is called, a few hundred Males, and thousands of Neuters, living together in one community. The proportions are somewhat different in other spe- cies—the Wasps-Humblebees, &c. &c. These facts, which are well known to Entomol- ogists, and all those who have become acquainted with the growth and education of Bees, show that the ideas which are generally entertained about • the specific distinctions and the characteristics of species, are not altogether correct. It is not throughout the Animal Kingdom that 62 PROF. AGASSIZ S species consist of individuals of two kinds; and to know those two kinds, is not sufficient to form a correct idea of the species. There are species in which individuals of various kinds are combined together, and in which the combination, in pro- portion to the numbers which are constant, con- stitute an additional character of the species. And for those, we must enlarge our notion of specific limits, and introduce elements which are generally overlooked. But I proceed to the illustration of the class of | Crustacea. These animals constitute, as they are ] now circumscribed, a very natural group; though j it may be -\ ery difficult to assign general charac- j ters to it. And, indeed, on trying to find a practi- I cal trait of character, a combination of structural j peculiarities, which should exclude any other an- | imal, and combine together all the Crustacea, I have strongly felt that these animals were now combined as they are, not from any anatomical evidence, but from the very reason on which I insist as the foundation of classification; namely, j from various hints about the growth, the mode of | formation, and the transformations of their species. They are so heterogeneous in their external a*- j pect, as scarcely to indicate animals belonging to j one class. Who would suppose such a congrega I tion of large shells, (here the Professor exhibited a large bunch of Barnacles,) to be Crustacean— to have an animal allied, for instance, to the Horse- Shoe, or to the Crabs, Lobsters, Shrimps, and the like. Nevertheless, it is certain, from what we know of the metamorphoses of the Barnacles, that they, too, as well as many Worm-like Parasites. belong to the Crustacea. The importance of Embryological studies, for a correct understanding of the true character and elassificat-on of animals, is so plain and so ob- vious in the class of Crustacea, that I beg to be al- lowed to illustrate more extensively this class, in this respect, than I would otherwise. I would begin this, by pointing out some pecul- iarities in their form, which have reference to the ehanges which these animals undergo during their metamorphoses. Plate XVII represents various animals, all of which belong to the class of Crus- tacea. In Plate XVII, fig. A, is a Crab (Lupa dicantha) seen from above; and in Fig. B the same as seen from below. You may notice the number of legs. They are in pairs—the anterior pair of which con- stitute powerful claws; the others being termi- nated by a simple joint at the erd. The body is so contracted that the longitudinal diameter is shorter than the transverse diameter. It is a pe- culiarity of almost all Crabs, thasrlheir longitudi- nal diameter, if not shorter, is scarcely longer than the transverse. Another peculiarity is, that the tail itself is short and bent under the main part of the body. (Plate XVII, fig. B). The main body, which is neither a head nor a fPr.vTE XVU—Cn iti° ivn STTtMvpg T chest, but which is simultaneously both, and on that acconnt is called cephalo-thorax—the head- chest— contains the main mass of organs : the ner. vous system, the alimentary canal, and the heart as well as the respiratory organs, which are in these animals attached to the legs. This peculiar, contracted form will presently be found to have reference to some changes,which are noticed in the growth and metamorphoses of Crustacea; and are therefore essential, On the anterior portion of the body, there are thread-like appendages, called antennas or palpi Of those, there are two pairs ; one an inner pair, and the other an external pair; and sideways from those, are eyes. They are, in these Crabs, (Plate XVII, figs. A, B) placed in a little depression on the side of the shell, so that they cannot be seen in the position in which this animal is drawn in this plate. To see the eyes, we should look into the face of the animal. Between the eyes and palpi are the jaws, consisting of a very powerful appa- ratus of moveable appendages. The position of the main organs is the more im- portant, as it is reflected by the external covering; so much so, that from the outside, in various spe- LECTURES ON EMBRYOLOGY. 63 cks, the position of the heart can be recognized by definite outlines. These outlines in the shell (Plate XVII, fig. A) cover the position of the heart. This other outline indicates the position of the gills. This becomes possible, from the fact that those or- gans, though soft, are earlier developed than the shell, which is mqdeled over the organs. The position of the gills is important on one ac- acount, being always connected with the legs; though they appear to differ widely in their posi- tion in various Crustacea. Where they are cover- ed, they are attached upon the thigh, the shield extending over their point of insmion. In other Crustacea, the gills are external—they are attached to the external joints of the legs, and seem to present an entirely different connection from what wo observe in the Crabs. But the mo- ment we go to the bottom of the question, we see that here also the respiratory organs are connect- ed with the leg, only that they are from the upper portion, and covered by a shield, as it is devel- oped. In those Crabs, the nervous system presents a very interesting arrangement. Above the alimenr tary canal there is a first mass, which gives threads for the head proper, a kind of brain; a ring around the alimentary canal connects this swelling with the other swellings; but these posterior swellings form only one uniform mass in the centre, from which threads go to all the rings of the chest and . their appendages. And this position, this concen- trated arrangement of the nervous mass, is observ- ed in all Crabs. In other Crustacea, the nervous centres under the alimentary canal are more or less scattered, and correspond directly in their po- sition to the rings which they furnish with nerves. This structure of the nervous system plainly shows that the Crabs roust be conndered as ranking highest among Crustacea, if we remember what has been observed in the Caterpillar, in which, during its metamorphosis into a higher form, the nervous swellings were observed to concentrate gradually more and more into compacter and fewer masses. [Plate VI page 41—Lobster.] Let us now compare these Crabs with a Lobster, (Plate VI) or with a Shrimp, (Plate XVII, fig. C) a species of Shrimp which occurs in the Southern States, called Peneus setifer. The general arrange- ment of the parts is the same as in Crabs. Here we see first the cephalo-thorax covering the main organs, and the anterior pairs of legs, covering also the mouth, and from which, on the anterior part, arises the peduncle for the eye and those appendages called the palpi. Next, we distinguish the tail, which is continuous with the head-chest, and forms a large part of the body ; a portion of the body, which is as large as the cephalo-thorax, or even larger, and which can be curved forwards, but which is never permanently bent under the cephalo-thorax. Such an arrangement of parts is also observed in the Lobster, (Plate VI) which does not differ materially in its structure from what we have noticed in the Shrimp. The various rings which constitute this cephalo-thorax and the tail, are equally provided with moveable appen- dages, which are represented separate in Plate XXI. In the head we notice a short peduncle, (Fig. A) terminating with a compound eye.consist- ing of thousands of little lenses, each of which has a crystaline lens, a nervous thread, and really is a compound eye. Next, we have those two sorts of palpi represented in figs. B, C. Next we have six pairs of horizontal moveable jaws, (Figs. D, I,) three of which are more powerful perhaps than the others, and constitute what are called the jaws (Figs. D, F); whilst the three others are called jaw- feet, from their close resemblance to the logs in many of these animals. [Pr.iTF XXI—Appendices of ^nrs-rACF*.] The three first pai», which are near the palpi are properly called jaws; and the three following pairs are cajled jaw-feet, (G, H, I). They are call- ed jaw-feet, for having internally, like the legs proper, appendages which are modifications of the apparatus which supports the gills proper. These appendages, however, (Figs. G, H, I,) instead of being complicated gills, have only fringed mem- branes, extending backwards, without performing respiratory functions. So that, in these parti which surround the mouth and act as jaws, we have the same connection between the respiratory organs, as that wc observe fn the legs under the chest. 64 PROF. AGASSIZ7S So that, notwithstanding the functions of these parts, whieh are used to crush food before it passes into the alimentary canal, we see that they are a modification of the same appendages which on the side constituted siniple legs. Here, jaws and legs are really modifications of one and the same type of appendages. But the chest is not one continuous mass, (Plate VI). It consists of several rings, united in the fall grown individuals, but distinct in the young, and still distinct on the lower surface of the adult These transverse ridges, (Plate XVII, fig. B) which are noticed between the legs, indicate the Mints, which, by their re-union, constitute the chest, or cephalo-thorax. And so we cannot wonder that there are as many pairs of legs as there are joints united to form the cephalo-thorax. These five pairs of legs of the chest are figured separately, (Plate XXI, figs. I, K, L, M, N). But, are we allowed to consider the cephalo-tho- rax as consisting simply of five joints, and one for the head 1 If it be true that every joint can have but one pair of moveable appendages, then we must admit that the head, however eontracted, is the result of the re-union of nine distinct joints. The eyes, the palpi, the three pairs of jaws, and the three pairs of jaw-feet. And indeed, so many transverse divisions may be noticed in the interior of the chest, in its anterior extremity, when ex- amined closely; it can scarcely be doubted, there- fore that it is out of so many joints that the cepha- lothorax has been formed. At the posterior part of the body, under the tail, we have other appendages, which assume the shape of branched threads, as represented in Plate XXI, figs. O. P, Q, R, S. These are modified legs, which are not used in locomotion, but to which the eggs become attached when they are laid, and as they remain suspended to the lower side of the tail, they are carried about by the female Crabs till the young are hatched. The fin-like appendages at the extremity of the tail, (Plate VI), are still other modifications of legs; and so, throughout the longitudinal axis of such an animal, whatever shape its body assumes, whether in Insects or Crustacea, the appendages used as legs or as jaws, are only modifications of one and the same sort of organs. It was important to come to this conclusion, in order to be allowed to compare the various appen- dages which were noticed on the side of many of these other Crustacea, (Plate XVIII). For in- stance, in Squilla, (Plate XVII, fig. D), we have a kind of claw, of a very different nature. It is no longer as we see it in the Crab, but it is the ter- minal joint which is bent over the preceding one. j So that the claw here would resemble the motion of my arm pressing against the shoulder, and forming a forceps, not by the antagonistic action of two articulations moving against each other, as in the Lobster, but by the bending of the last joint ! against the preceding one. Many other modifications of these appendages are noticed on the sides of the body of Articulata; but the time will not allow me to give all these de- tails ; I merely refer to them for the sake of further comparisons. Let me only show that here in Sto- mapoda or Amphipoda, there is a difference of ar- rangement in plate XVII, fig. D, and plate XVIII, different from what we have in Crabs and Lobsters. The gills are entirely internal in Lobsters and Crabs; in the Squilla they are below the rings. Is there an essential difference in such a position 7 No, there is not. If we look at the embryo Crawfisb,as it has been figured by Rathke, we shall know that the shield, or the external covering, is gradually modi- fied by the development of the shield, whieh grows successively over the gills. The gills are external where they are attached to the tower joints of the legs, and are not different in their nature, bat only modifications of one and the same type. All the Crustacea belonging to these two groups. or rather to these three groups—the Crabs, the Lob- sters, and the Squilla—are among the larger of the class. The other types, represented (Plates XVHI, XIX, and XX) are almost universally small—some even microscopic. In the Amphipoda (Plate [Plate XVIII—Low Species op Crustacea.] LECTURES ON EMBRYOLOGY. 65 XVni, figs. A, B, and C), we have a structure re- sembling the Shrimp in its general outlines; but in the eye, we have no longer a peduncle. The eye is sessile—that is to say, it does not rise above the surface of the body upon a peduncle. In the others, Decapoda and Stomapoda, the eyes proceed from a moving peduncle, and are provided with the peculiar apparatus for seeing.— Such eyes are, therefore, moveable upon the joints of the peduncle; but in these Amphipoda the eyes are flat upon the shield (Plate XVIII, fig. B). You see that there is a diversity of legs among them, and a peculiar kind of claws in the anterior part- various appendages performing at the same time the function of legs and gills, and the tail similiar to that of Decapoda (Plate XVII, fig. B). One modification, however, will strike you. There are no longer many joints united to form a cephalo- thorax, but all the joints are nearly equal. The head constitutes only a joint similar to those of the rest of the body. There is no concentra- tion of legs in distinct regions. The number of these animals which occur in this vicinity is very great'; but they have, by far, not all been described- A few only have been mentioned in Dr. Gould's Re- port. Even genera which have not been described at all, occur in the harbor of Boston. Here, for instance (Plate XVHI, fig. C), is one of the new species, a new generic type, which is very beauti- ful. It Is a curious fact that among these animals there is such a variation of color. I have had a good many of them drawn and painted, in order to collect all the variatioas of colorations which exist It is scarcely possible to find two specimens which agree in color ; and many differ in the distribution of color so much, that if they were brought from different countries, and if it was not known that they lived together, Naturalists might arrange them as different species. In various individuals of the same species, (Plate XVIII fig. A) we find some are red, and others (Fig. B) green, others bluish, and others still, with every variety of color. To this fact I shall call again your attention hereafter. We have (Fig. E) others still different, in which the different joints are so slender as to form an elongated figure with outward appendages to it.— The middle appendages are very simple; the anterior ones have claws, while the posterior ones are mere simple legs. But on the whole, they come near to the Amphipoda, (Plate XVHI, fig. A.) As the legs, however, show some modified combinations, they have been considered as a peculiar family. under the name of Loemodipoda. In some Crustaceaof another form, (Plate XVIII fig. D) the rings are also not combined in distinct regions, and the eyes arise equally from the level surface of the shield; but the legs are uniform, and the uniformity goes on, increasing as we proceed lower down, to the various forms of this type which comprise the Isopoda. All the Crustacea of which I have spoken, have one common character—a thin calcareous shell; [Plate I—Germs of Scorpion.J whence their common name of Malacostraca is de- rived. Those of which I am now to speak are dif- ferent in this respect, and have been called Ento- mostraca. Some of them (Plate XVIII, figs. G ano> H, and Plate XX, figs. F and L,) are Parasitic Animals, in which we observe two long ap- pendages, or ovaries, hanging down from the posterior joints. The body in the Entomos- traca is simply protected by a horny shield or envelope, lining the back. There are some (Fig. G) in which the body is elongated, in the shape of a Worm, and in which the joints are almost en- tirely gone; so much do they differ from the com- mon character of Crustacea; and indeed.in such an animal as the Lernea, (Plate XX, fig. L) there is no joint at all to be distinguished; there are not even gills to be observed; there are no legs to be found in any part of the body; there is no heart; no one of the leading anatomical characters of this class of animals is observed in the Lernea; and nevertheless it is a Crustacean. It is one of those Crustacea which have been long known in their later condition of life, when they have be- come attached Parasites, but which have not been known in their earliest stages of life, when they are free, moving, independent individuals, with all the characteristics of other Entomostraca and similar Crustacea. These young, however, have the structure of Crustacea, inasmuch as they have fringes, appendages to their rings; inasmuch as there is a nervous system, presenting the arrange- ment of the nervous system in the Cyclops. But, when they have been freed for a certain time, they become attached, and are then Parasites, and un- dergo a most remarkable retrograde metamorpho- sis, by which they lose all the peculiarities of their structure, sink to a lower condition of life, and producing a great number of eggs in this condi- tion, finally die by a peculiar kind of bodily de- cay, as it were, which we nevertheless cannot con- sider as a decay, as it is in this curious stage of these animals ithat their eggs are most rapidly produced. It is really, as Rathke has considered it, a true retrograde metamorphosis in after life. But it is remarkable that there should be animals be- longing to the class of Crustacea, which have so entirely lost the aspect of Crustacea; which have no one of their anatomical characters, and which, nevertheless, belong to that class, as is shown by their metamorphosis. 66 PROF. AGASSIZ S [Plate XIX—Young Crabs, Shrimps, Barna- cle AND CYPRI8 1 to a certain size, becomes attached, and is trans- formed into the remarkable Barnacle. Here are some more, of the curious Entomostraca (Plate XX) to which I shall call your attention. We have (Figs. A to E) one species. (Figs. F to K) another species. This latter one, resembles the Lernea in many respects ; being attached by a sucker to the gills of fishes, on which they live, but having still a proboscis with jointed appendages, and having also indications of rings in the posterior part of the body, and having sacks of eggs hanging be- hind. In the other, Apus,(Figs. A to E) the body Plate XX—Rotifera. and Parasitic Crus- tacea ] We may say the same of Barnacles, in which in the final condition there is nothing of Crustacea in their external appearance; but which when young resemble common Shrimp-like Crustacea, to a very great extent, as we see by comparing a Cypris. (Plate XIX. fig. F, with a young Barnacle, fig. G). There are several of these horn-shelled Crustacea which have been described as peculiar animals; for instance, the species figured, which constitute the genera Foda, Megalopa and Cuma, (Plate XIX, figs. A, B, C, D, E,) which are nothing but young Crabs and Shrimps. Their resemblance to Cyclops, or Calanus, (Plate XVIII, fig. F.) or to Cypris, (Plate XIX, fig. F) is however striking. Here is a species (Plate XIX, fig-F) of Cypris, for instance, which resembles, not only the other young Crusta- cea of figs. A, B, C. D, E, but even the young Barnacles (Plate XIX, fig. G) most remarkably. The young of a shelly animal, which in this early condition of life is a little, free, moving Shrimp-like Crustacean, with an elongated tail, with legs and respiratory fringes, having eyes in the anterior portion of the body.which is similar, in fact, to other young Crustaceans, and which, after it has grown j LECTURES ON EMBRYOLOGY. 67 is free through life; but all undergo similar changes in early life. The Horse-Shoe Crab, though large, and in many respects somewhat more complicated in its struc- ture, belongs also to the Crustacea which have not a calcareous, but a horny shell, and are called Ento mostraca. From these facts, you may observe that Natural- ists divide the Crustacea into two great groups; those furnished with a shield, like the Crab and the Lobster, called Malacostraca, and such as are not thus protected,called Entomostraca, which have only a horny envelope, and in which all the parts are less diversified. I may mention more particularly one of these Entomoatraca (Plate XVIII, fig. F) a species of Ca- lanus, which has a peculiarity of being phosphor- escent, and of presenting a peculiar kind of phos- phorescence which I am not aware has been ob- served before Here the nervous system, with the eyes,is the shiningpart of the animal; that nervous system being not only phosphorescent, but the substance of the nerves being of a highly red col- or. The arrangement of the parts is precisely the same as in the nervous system of the Crusta- ceans in general. A close investigation of this arrangement ha3 shown me that there can be no mistake about it. [Plate XXII—Eggs of Pinnotheres.] mm i The embryonic growth of Crustacea has been extensively studied. We have had numerous mo- nographic investigations upon that subject, which were made by the most eminent of the Embryolo- gists of our day. Rathke, in particular, has in- vestigated that subject to a greater extent than any one else. However, the earliest changes which the egg undergoes, have not been so completely exam- ined. Therefore.allow me to call your attention for a few moments to the transformations of the eggs of the little Parasitic Crab, the Pinnotheres Os- trium,which is found in Oysters, and lives as a Par- asite between the gills of this animal. The whole animal is so transparent that its growth and changes can be very easily investigated. And there we find eggs of various degrees of develop- ment, some exceedingly minute,which consist of a simplely vitelline membrane.with an absolute trans- parent yolk, a small germinative vesicle and a ger- minative dot in the centre; a few granules are no- ticed in the yolk substance. • Others will present the same appearance in general structure, when the germinative vesicle will be much larger, and the germinative dot also much larger, being swollen into a small vesicle. The same will be universally observed in a series of changes, where we notice that the germinative dot may grow much larger than it was before, and even form a hollow vesicle within the germinative vesicle it- self; the yolk granules having greatly increased in quantity between the germinative vesicle and the vitelline membrane. So that here it is perfect- ly plain, that, the germinative dot can grow into a hollow vesicle ; and from the condition of other eggs, we may be satisfied that there is a period when the germinative vesicle and the germinative dot may disappear, to give rise to the formation of another germinative vesicle containing more nu- merous granules; and that that vesicle may burst again.and give rise to the formation of two germi- native vesicles with their germinative dots, or we may have three germinative vesicles with their germinative dots. And during this period of evo- lution of cells within cells, there is an increase of the mass of yolk taking place, an accumulation of granules growing, by which that egg finally as- sumes that degree of maturity, which precedes the first formation of a germ. [Plate III—Eggs and Development of Shrimps ] I have traced these eggs up to the moment when the yolk had become a mass of somewhat opaque, though not very compact yolk, and the first rudi- ments of an embryo were: formed, as a disc on one side of the egg, growing around if. and pre- senting all the changes which have already been described by Rathke and Erdl, as occurring con- stantly in the growth of Crustacea, and to which I will now allude, referring to the species which he has figured. The earliest condition of these germs in Palae- mon, (Plate III. fig. A,) after the egg itself has un- 68 PROF. AGASSIZ S dergone all its changes, is the formation of a lay- er of more animated substance, the beginning of the young animal. We have here (Plate III, fig. B) the germ as it flattens out at one end and is contracted at the other part, divided as it were in- to two connected discs, the larger assuming after- ward another form (Fig. C), the smaller one grow- ing laterally, when soon it is observed what has be- come of these two extremities of the expanded disc (Fig. D). One will be the head end of the germ, and the other will be the caudal end of the germ. Those serratures upon the posterior ex- tremity of the animal, represent the divisions in the animal layer, in the blastoderms, or germ, which will give rise to the joints or rings of the chest; while the anterior disc will represent that part of the body which properly forms the head, growing larger and larger; these flat discs are drawn backwards, forwards and on the side, so that it gradually surrounds the yolk, having assumed a more elongated shape (Fig. F) leaving the mass of the yolk free at the dorsal side, so that when seen from above (Fig. G), you have the margin of the animal in sight, which is rolled over the yolk. We have also here the eyes, which are forming at the anterior portion of the germ ; and also the in- dications of the formation of a heart. But from below (Plate III, fig. E,) we see how the lower surface is changed; the formation of those parts which will represent the mouth, is seen, and also the formation of those parts which will represent the legs, and in addition, the parts which will represent the tail. And those separ- ations of different joints become gradually more and more distinct, (Fig. G,) so that upon close examination,you may find that the germ is now a little animal, which soon escapes under the form of fig. H. Here we have the young, which rises from such a transformation; and this young is the young of a Palaemonof the character of Plate XVII, fig. C. The young as it is hatched represents the figure which is a general characteristic, not only of the Macrouran Crustacean, but it has more particularly the form of those Entomostraca which have been described under the name of Cuma (Plate XIX, figs. D and E) I have traced many of those which occur in Boston harbor, of Palaemon, of Hippolyte, even of Mysis, and they all give rise to young which are species of the genu3 Cuma, belonging to the Entomo- straca of Carcinologists ; showing that there are still extensive grounds to cultivate in the history of Crustacea, and that they undergo metamor- phoses The subject of the metamorphoses of Crustacea has been discussed very extensively, Rathke denied positively that there are metamor- phoses among Crustacea; while facts were col- lected in Ireland which showed distinctly that such metamorphoses take place. Mr. J. V. Thompson, who has published many interesting investigations upon the lower Marine animals—the same to whom I have before referred —and who discovered that the young Comatula had a stem in its earlier condition, was also the first to notice that the so-called Zoea (Plate XIX fig. A and B), were not animals of a peculiar ge- nus, but that they were the young of Crabs—of Crabs of similar form to that figure, (Plate XVII, fig. A.) Captain Tuckey of the British navy, ob- served similar changes. He saw the transforma- tion of the egg into those entomostracal germs,and further changes, which left no doubt in his mind that the Crabs underwent the above described met- amorphoses. The objections of Rathke arose from the fact,that the Crawfish, a Crustacean, in which he studied that embryology, does not undergo extensive chan- ges of form during its embryonic growth. The young Crawfish resembles very early the perfect animal; so that by correct investigations this emi- nent Embryologist was misled; though he after- ward acknowledged his error with reference to the investigations of Thompson, in the most liber- al and generous manner. These metamorphoses have been traced extensively in other Crustacea. Zaddach has published a monograph, in which he has represented the changes which this animal, Apus, (Plate XX, figs. A to E) undergoes, from its primitive formation in the egg, up to its perfect condition, (Fig. E.) In the beginning (Fig. B) it has but few appendages ; and afterwards, others successively, more numerous, are added under- neath. Here (Fig. F) is a diagram of another ani- mal, the Achtheres, in which similar embryonic changes have been observed. First, there are also but few appendages, but afterwards several pairs have been added to form the various appen- dages which exist in the adult (Fig. G.) How similar Rotifera are to these various embryonic conditions of Entomostraca, will not escape the observer, who is simply reminded of the existence of these microscopic animals, (Plate XX, fig. 0.) They resemble most remarkably those Entomostraca in their earliest condition. But in their embryonic condition,- Crustacea — even Crabs, as well as Lobsters have young which re- semble perfect forms of those Entomostraca, be- yond which certain Crustacea do not pass. We have thus direct indication that they should be con- sidered as the lowest; and so would we place at the lowest range, all the Rotifera and these vari- ous kinds of Entomostraca and Parasites, (Plate XX and Plate XVIII.) Next, we would have the Malacostraca; and among them, those lowest, with uniform rings, which are not combined into dis- tinct regions; and next, those in which the rings are also not combined, but the legs diversified, (Plate XVIII, figs. A, B, C, E); and above all, those in which the rings are combined in various ways, which are still more diversified, (Plate XVII); placing the Lobster and Shrimp lower among them; but we should consider the Crabs (Fig. A) the highest of all, because in these the concentra- tion has gone to the extreme, the tail which was LECTURES ON EMBRYOLOGY. 69 proportionably the greatest appendage, the longest and most developed part of the body, in the earli- est condition, being now reduced to the simplest and lowest condition. Such a classification agrees with the classifica- tion which has been introduced into our natural histories, from the general impression received from these animals. Guided to some extent by anatomical details, and also in some points by em- bryonic data, the arrangement proposed has been the same to which, from embryonic evidence, we would arrive. Only, there is an objection to be made to the division of Crustacea into two groups; Entomostraca, passing by transformation into Ma- lacostraca, as can be directly ascertained in the case of Cuma,theyoung Palaemon. Therefore, that division cannot stand as a natural division. We must have a series of groups following each other, according to their embryonic gradation, but not two types of Crustacea; as the differences upon which this distinction rests present only degrees of one and the same thing. But, there is another point in which the analogy of gradation with embryonic growth is most re- markably striking. It is the order of succession of Crustacea in geological times. Crustacea have existed from the earliest times. They are found in the earliest formations, and found in all subsequent beds. [Plate XXIII—TriloriteI The forms assumed are different. The oldest are the so-called Trilobites of several types (Plate V). There is a remarkable analogy between the forms of various Trilobites, and the outlines of the germ of Crustacea, as figured Plate III, the earlier stages reminding us of Agnostus, and the like, whilst the later agree more with the higher Trilobites; but the most striking resemblance is noticed on comparing these types with the embryo of the Entomostraca, as they are represented (Plate XX, fig. A) within the egg, before they are hatched; the divisions of the middle part of the body into three lobes, the long, lateral appendages arising from the anterior extremity. Every point of the structure agrees. It is only, that in these ancient types there was a permanent state of growth—a condition under which this animal lived for ages, and reproduced its species; whereas, in our lowest Crustacea we find even such an arrangement in the earlier form only, as the beginning of a metamorphosis. Next, we have in the geological series, Horse-Shoe Crabs. During the coal period, there existed seve- ral genera of Crabs allied to the Horse Shoe,having the same general features. There are also species found in the Oolitic beds. If we trace the grada- tion of types, we find that these (Plate XX fig. A) the Apus, in their perfect state, are next in order. Those which undergo a retrograde metamorphosis or which agree with the embryonic stage of Apus, as Trilobites, being altogether the lowest. And so we have the Horseshoe Crab, which is the second type in the order of geological ages, ranking high- est among Entomostraca; that is,above those which resemble the Trilobites. During the deposition of the Oolitic and Creta- ceous rocks, there existed a countless number of Crustacea, but all of them were Lobster and Shrimp-like animals. The earliest of all the Mala- costraca is a long tailed animal, the Palinurus Sueurii, resembling Lobsters and Shrimps. And tduring all this time, we have only such animals— and not one Crab is formed until afterwards. But, during the later part of the deposition of chalk.we begin to find Crustacea with short tails, belonging to the type of Crabs. So that, in the order of suc- cession of the more recent types, we have the same evidence that the arrangement which is proposed, from embryonic data, is also the order of progress which has been introduced into the character of these animals at different successive periods. And I may add h$re, that the geographical dis- tribution corresponds even to this gradation of types, as far as it is understood. Crabs, for in- stance, are not numerous on this shore. Few spe- cies occur here. In the Middle States they are more numerous. They occcur more frequently and are very diversified in South Carolina; and still more numerous, in the tropics, where Crabs prevail over Lobsters and Shrimps. And, though these latter are extensively found in temperate regions, it may be said, that the lower orders of Crustacea (Plate XVHI, fig. A) are innumerable in the northern regions, and much fewer in the trop- ical regions. So that, in whatever point of view we notice this subject, we see one plan, one com- bination, one system, uniformly carried out. 9 70 PROF. AGASSIZ'S LECTURE IX. More than once I have alluded to the uniformity of structure of the egg, in its primitive condition, in all animals; thus showing that there is a com- mon starting point for their growth, throughout the various classes of the animal kingdom. I shall now illustrate more fully the physiolog- ical process by which the egg, when matured, gives rise to the formation of a germ. I do not intend this evening to enter into more details than I have already given, upon the formation of the egg itself, but to illustrate the process by which the egg gives rise to a germ. This process has been traced in all classes of the animal kingdom; and it is found to consist of a very complicated se- ries of changes taking place in the substance of the yolk, when it has reached a certain degree of ma- turity. The condition, therefore, the first essential and constant condition for the formation of a germ, is the previous formation of an egg, and its beingi matured to a certain degree. The size, the degree of maturity, and changes which the egg itself un- dergoes before the germs are formed, vary in dif ferent classes. I will not allude to that point at all, but only take now the germ as it is forming within the egg, when the yolk has grown to a cer- tain size. [Plate XXIV—Eggs of Planarim ] eggs are thus contained in a transparent sub- stance of shapeless appearance. After the laying of the eggs, another series of transformations is produced, as we shall see pres- ently. Almost the same changes occur in the Malacobdella, which is a Parasitic Worm found in the Clam. There, also, we have observed yolk bottles, as also the successive formation of the eggs. Here there is no ovary proper; we have found the bottles distributed in the whole body around the intestinal canal. Some contained only one egg, and some not yet condensed yolk sub- stance ; others contained two eggs; others three, four, and even a greater number were formed, until the whole yolk was exhausted. Plate XXV represents some of these phases.— In the Planariae the mode of formation of the eggs is the same, except the bottles. [Plate XXV—Eggs of Malacobdella] I cannot, however, onm mentioning a very curi- ous mode of ovulation which is noticed in some Worms. When, some months befoi e the laying of the eggs, we observe the ovary of the Nemertes, we see in their interior, oblong, bottle-shaped pouches forming, which fill with yolk substance, that gives rise to the eggs. When these bottles have attained their whole devejopment, that is to say, when they are completely filled with yolk substance, a new process is introduced in them.— The substance groups itself around several centres, and forms a series of little spheres, whose number varies. These are the eggs; eggs which soon have a germinative vesicle, and within it, a germi- native dot characteristic of the eggs in general.— When this second progress is terminated, the bot- tles are laid, under the shape of a chain, and the Let us return to the egg, when it is about enter- ing another series of changes. In Piates XXIV and XXV, we have eggs of different animals, in which the process of the formation of the germ is represented up to a certain degree of its growth. The primitive egg consists, as you re- member of a vitelline membrane containing yolk, and within this yolk a germinative vesicle,and with- in that a germinative dot, as shown in Plate XXIV, A, B. The yolk becomes gradually more and more condensed, thickened, and more and more opaque ; and at that epoch, the germinative vesi- cle generally disappears ; the germinative dot dis- appears also, and new changes begin to take place within the yolk. It has been questioned, whether the germinative vesicle and the germinative dot precede, or follow the formation of the yolk substance. There are examples of ovarian eggs in which this vesicle and this dot are very distinct, as also the yolk mem- brane, at the time when the vjtellus is yet very thin and transparent in the sphere of the egg. We have seen this vitollus increase and fill up the whole LECTURES ON EMBRYOLOGY. 71 space and condense around the germinative ves- icle. So that there was no more possibility of doubt that the vesicle and the germinative dot did exist there before the vitellus. At other times, the Kerminative vesicle alone has been observed in the developing eggs. There are other instances where the ovarian egg presents neither germinative ves- icle nor germinative dot during the formation of the yolk. This shows that even the question of the fundamental structure of the egg, in order to he solved, calls yet for minute and serial research- es. In the interstices of the granules or little cellules which compose the vitellus, is contained a transpa- rent liquid more consistent than water, since it re- sists a certain pressure. When the egg is formed this liquid tends towards a centre and agglomerates itself there under the form of a transparent sphere, the appearance of which precedes the ordinary phases of the dividing of the yolk. Whether the progress is the result of the mix- ture of the contents of the germinative vesicle and the germinative dot", or the changes are intro- duced simply owing to the fact that the egg has arrived at its maturity; whether it relies simply upon the yolk to undergo those changes, is a point which it is impossible to decide at present. Gen- erallyj when the yolk undergoes the first change by which the germ is formed, the germinative vesicle and the germinative dot have already dis- appeared; but in some instances, the germinative vesicle and the germinative dot have been ob- served within the yolk, when another mass, (the clear sphere) which generally appears after those have gone, had been formed in another portion of the egg, as represented in Pi. XXV, fig. H; so that changes which have been known to be connected with the first formation,—changes giving rise to the germ—such modifications are observed in the yolk when the germinative vesicle is still within. Therefore, it cannot be absolutely said that the bursting of the germinative vesicle, and the mix- ture of the substance contained within it, is prop- erly the cause of the changes now taking place.— It may have an influence upon the yolk, by which those changes are accelerated or facilitated; but tfiat it is properly the cause, cannot be main- tained. Well, to understand all these changes which take place within the egg, they must be conceived as successive modifications of substance We know that one sort of egg will only give rise to one sort of animal. Therefore we must admit, that as an egg of one kind gives rise only to one sort of ani- mal, there must be an immaterial principle presid- ing over these changes, which is invariable in its nature, and is properly the cause of the whole process. But now the changes which take place in the yolk vary in different classes of animals. In some they consist of a division of the yolk, which is successively repeated and repeated, till the whole mass of the yolk has been so much subdivided as then to consist of innumerable little masses, aris- ing from the subdivision, from the repeated subdi- vision of the primitive mass into successively more and more numerous parts. In others, the division is only partial. On one side of the yolk there is a depression formed, which does not penetrate across the whole mass, and then another, which will be formed at right angles with the first, thus forming four partial divisions; and that being repeated, the surface of the yolk, on one side of this mass may be divided into little fractions, though a great portion of the yolk takes no part in this process of repeated division and subdivision. In many ani- mals the division of the yolk is most wonderfully regular. The dividing of the yolk is probably a general phenomenon, appearing in all eggs, though obser- vation has not revealed it to us in all classes with the same certainty. Its generality, however, is difficult to trace at present; as its various modifi- cations have not been reduced to one common type: however, the fact is already ascertained in the class of Mammalia. In the Birds, the size of the eggs has been an obstacle for this kind of ob- servation. It has been noticed in the class of Rep- tiles, and in that of Fishes. I have already men- tioned the difficulty which observations encounter in the class of Crustacea and Insects ; in regard to which the data upon the dividing of the yolk are deficient, although it has been observed in the in- ferior Crustacea. It is easily traced in the Worms and Mollusca; indeed it is nowhere easier to ob- serve it, than in these two classes of animals. The phenomenon of dividing of the yolk does not fol- low the same course in every class at the same stages of dev elopment. Perhaps it begins, in some cases, even before the laying of the eggs. This would explain, at least, why it has sometimes not been observed. The process is sometimes slow, sometimes very rapid; and in this latter case it may easily escape the attention of the observator. Nor must we lose sight of the fact that embryogenic science is a comparatively recent one, and in this department there remains yet much to be done—above all, with reference to the study of tis- sues. This should especially be acknowledged, if we consider that it is as late as the year 1834, when Schwann made the discovery of the uniform cel- lular stiucture of organic tissues, in the animal as well as the vegetable kingdom. There are animals, (and it has been more par- ticularly observed among Worms, among Intestinal Worms especially, by Dr. Bagge,) in which the yolk first divides into two halves, which subdivide and subdivide regularly till the whole mass of the yolk is reduced into minute uniform yolklets. The process of this division is also seen in Mollusca, especially among naked Mollusca; the whole mass dividing into two halves, forming two distinct masses. Next, each will be subdivided into two 72 PROF. AGASSIZ'S so that the primitive mass of the yolk will be divided into four equal parts. And then those segments will be subdivided and subdivided, till the whole mass consists of small yolklets, each surrounded by a membrane. But the subdivision is accompanied by a pecu- liar formation of other masses within those partial spheres. Let me show yon some diagrams repre- senting this process. In Plate XXIV, fig. C, we have the eggs of Planaria,in which the yolk is divi- ded into four masses; and in Plate XXIV, Hg.Tf, we have it the same under slight pressure, when four clear spheres are notieed within each of these seg- ments. In the next place we observe that besides the four great masses there are four small ones, rising in the centre. Again we may observe in each of the small ones such a clear sphere, and when the subdivision goes on forming a greater number of these spheres, the whole process is repeated, the large one being greatly reduced, there being successively, 16, 32 or more. Such fragments are increased very reg- ularly, and though many variations are observed, they appear in multiples of two or four, and so on. When it has gone on a certain time,instead of four small ones and eight large ones, or vice versa, there will be quite a number of minute ones, and all alike in size, and the process will be repeated till these divisions are so minute that it is no longer possible to count them, they forming a mass of little cells, filling the whole of the membrane of the yolk. What those clear spheres within the yolk are, it is somewhat difficult to say, inasmuch as chemi- cal analysis cannot reach them. The eggs are so small that their composition has not been exam- ined. It is only with the microscope that we can reach these processes and determine the changes of form and substance which take place, by the various properties of these substances with refer- ence to light. The fact of their being more or less transparent will make some appear different, under the mi- croscope, from others. And that is the whole ground upon which the ehanges can be ascer- tained. The manner in which the division takes place when there are two forming, for instance, in the intestinal Worms, has been described by Dr. Bagge, as follows. The primitive clear sphere in the centre is said to assume an elongated form, and then the centre to be contracted, and finally the two ends become independent by a separation of the middle part, so as to form two spheres; and then the yolk mass to agglomerate around those transparent spheres; and then a division to be formed in the vitelline mem- brane ; and that to go on and to divide the vitellus into two spheres ; and in each the same process having been repeated, to have transformed that into four. Assuming again an elongated form, and then dividing completely, thej go on and fonn four masses. But that clear spheres within do not always constitute or determine the separation of the substance of the yola into more and more nu- merous masses, is shown by the example which 1 have quoted, where a clear space exists in the cen- tre of an egg, and the division takes place across it. For instance, there will be such a mass as rep- resented in Plate XXV, fig. H, and the division will take place, a clear sphere accumulating on one side of the mass, and the yolk condensing on the other side, and so on. The fact is, that the subdivision of the yolk mass and the formation of these clear spheres, is a pro- cess which goes on simnltaneously, but which can- not be considered as directly dependant on each other. In proportion as this tendency of the yolk to subdivide is manifested by a contraction of the mass, and the division of the spheres into two spheres, in the same proportion the substance within the yolk, which fills the space in the centre of the yolk, accumulates in spheroid masses, to give rise to partial spheres. And that being re- peated, there are then numerous divisions of the yolk successively introduced, and having been en- tirely kneaded, as it were, by this repeated divis- ion, the substance of the yolk in process of time becomes a germ. For instance, in the Worm from which the dia- grams in Plate XXIV are made, the germ (Fig. A.) is a mass of very minute cells. Then from the surface of those cells rises vibratory Cilia. We know that cells can have vibrating Cilia on one of their extremities. It is observed in the full-grown animals, and it is observed in many germs, es- pecially in Mollusks, that such vibrating Cilia, are formed on the external surface of cells and become an apparatus for locomotion, which Cilia are voluntary, ceasing to move at intervals, re- newing their motion at other times and transport- ing the animal from place to place. But remark- able as it is, that the sphere is the fundamental form of all animals, so rotation is the form of the action of all animals when they begin to move within the vitelline membrane No sooner has the little Planaria (Plate XXIV) been covered with vibrating Cilia, than it begins to revolve upon itself; it has then a spherical out- line, and undergoes a rotatory, constant motion in one direction to begin with. And when it has grown to assume a somewhat elongated form, by which the prevailing longitudinal diameter will be introduced, after that longitudinal diameter has exceeded the transverse, then it will change the di- rection. And as soon as it is hatched, then it will proceed in an onward and forward motion, which will be the motion that will characterize the an- imal; and then comes the bilateral symmetry which exists throughout the animal kingdom, even where it is concealed under the radiated form of so-called radiated animals. A remarkable comparison might be instituted LECTURES ON EMBRYOLOGY. IS between the embryogenic phenomena, as we have just described them, and what is known of the ce- lestial bodies, in their combinations, upon an im- mense scale. First, we have primitive cells, com- bining and condensing to form the mass of the egg, like clusters of nebular stars. After the yolk has undergone the various phases which precede the formation of the embryo or germ, this new be- ing with a spherical form, which is also the form of the primitive egg, begins to assume a rotatory movement, under the influence of life, as the ce- lestial bodies rotate under the influence of univer- sal gravitation. At last the progressive, onward movement is introduced, which characterizes ani- mal life properly, and is the first step in the series of progress, which, in man, ends with intellectual freedom and moral responsibility. But this form of the division of the yolk is not the only one which is observed among animals. In Fishes, for instance, we have a division of the yolk, which differs considerably from that just de- scribed. In these there will be first a transverse depression upon the yolk, so that, seen from above, the yolk will seem divided in two halves. And then it will be divided again at right angles, so that there will be two furrows at right angles, forming a division which remains superficial. So that in a profile view these furrows do affect the yolk but very Httle, and the whole mass below re- mains unaffected. But only the superficial layer undergoes this change; the lower portion and the central parts of the yolk remaining unchanged, but being gradu- ally introduced into the process—being gradually absorbed by that part of the germ which is already formed, and finally totally absorbed by the germ ; or if not introduced into the substance of the germ as a part of Us body, it is finally introduced as a sac from the lower part of the body into the di- gestive cavity, and is digested. So that we have ail possible steps, from total division of the yolk, which is entirely changed into a germ, to a super- ficial furrowing giving rise to a germ which rests upon a modified yolk. In the first instance, by repeated subdivision, the whole substance of the yolk is prepared to become a germ ; or, in the sec- ond, only a part of it is modified to form a layer upon the yolk, which grows and gradually absorbs the remainder of the yolk. In those animals in which the division of the yolk is only partial, as in fishes, the divisions where they have been multiplied have nevertheless finally given rise to cells. In the beginning, those divisions are only separations of the superficial mass. But those masses not being entirely sur- rounded, do not form distinct spheres or parts of spheres ; but at last, when they have repeatedly multiplied, then each particle is surrounded by a membrane, and thus transformed into a distinct cell. So that the germ, in whatever manner it is produced—whether by total or partial division of the yolk—is finally, when formed, constituted of numerous small cells. The changes which those cells undergo—the manner in which additional cells are derived from the yolk, either by division or by evolution from those already formed,—con- stitute the phases of the embryonic growth of each animal. But it is by a uniform process of division that the germ itself is first formed. The degree of maturity which the germ has reached when it is hatched, varies extraordinarily. There are ani- mals in which the germ is hatched in a degree of development which is so distant from what the animal will be finally, that it cannot be recognized, and that the type of the parent is not at all indi- cated even in the outline, in the form, or in the structure of the germ when born. There are other animals in which, on the contrary, the germ is not hatched before it has grown within the egg to assume the external forms of the mature animal, and has even attained to a very considerable size, in many of them. It is perhaps from not having considered suffici- ently those differences that so many mistakes have been made in the study of the changes which those animals undergo. Had it been supposed that ani- mals were born in a condition in which they differ so widely from the parent, they might have been watched longer before they were described as dis- tinct animals, on the sole ground that they were free-moving. And we should not find that animals of the same species would be described under so many different names if this had been more gene- rally known. A great many larvae of Worms are undoubtedly simply those small animals described as Infusoriae; and I have myself seen eggs of Planaria give rise to some of these Infusoria called Paramaecium Annellides, Here, for instance, is one (Plate XXVI, figure E), remarkable for its sucker-like discs [Plate XXVI—Parasitic Worms.1 ly. (Plate XXVII, fig. B). The chaDge which 74 PROF. AGASSIZ S those germs undergo in various families of Worms seem to differ widely; and indeed, among Worms every where, there are types which are so widely different in their outlines as scarcely to afford char- acters by which to combine them. [Plate XXTII—Young Worms I It will be a great difficulty to find Anatomical as well as Zoological terms to constitute into one class all these various forms, (Plates XXVIII, XXIX and XXX) and those which are represent- ed there, (Plates XXXI and XXXII) Neverthe- less, in tracing the intermediate forms, we are compelled to bring them into one and the same group. [Plate XXVIII--Worms with Colorkd Blood J The class of worms, as I circumscribe it here, contains r.umcrous and very diversified types, as well by their internal structure, as by their exter- nal form; so that it is difficult to assign to all of them common characters. The Intestinal Worms, formerly considered as a class by themselves, can- not be separated from the true Annulata. There are intermediate forms between the two groups — For instance the Trematoda, which are closely al- lied to Planaria, the Ascaris, which resembles Lum- bricus, and so on. The Intestinal Worms, gener- ally speaking, have their body naked ; the Acan- thocephala only have hooks of fringe-like appen- dages. Among Annulata there are, however, types which cannot be compared with any of the Intestinal Worms; as the Tubulibranchiata and Dorsibranchiata. Among these there are some in which the lateral appendages of the body are uni- rPr.ATE XXIX—'Various Worms.1 form torus tvtiole length; mothers, the appen- dages of the anterior, middle and posterior region of the body differ among themselves, and assume even an entirely different character. In some, the rings are generally provided only with a few stiff hairs, whilst the head is surrounded with tufts of respiratory fringes, and other appendages, in va- rious degrees of development. Nevertheless, through all that diversity, there is a common type which can be easier understood than properly de- scribed or defined. The development of the class of Worms varies according to its types. In some, the yolk sub- stance, after having been indefinitely subdivided into homogenous little spheres or cells, assumes a rotatory movement, sustained by vibrating Cilia, which have been formed upon its whole sur- 1 face. Such arc the Planariss, &c, &c, whose LECTURES ON EMBRYOLOGY. 7.5 [Pute XXXI—Intestinal Wobm3.] young are lulusorisui;-. in others, the envelop- ment resembles more that of Crustaceans and In sects, there being an animal layer formed upon the lower side of the yolk sphere, which surrounds gradually the yolk and encloses it, so that the narle is dorsal. Such a growth has been observed in a worm of the L°.ech family, which occurs in Fresh Pond, (Plate XXXIII) as well as in a marine Worm of the bay of Boston, belonging to the ge- nus Pasithte. I wish only to make some remarks upon the va- rious metamorphoses which the Worms undergo. Among the Intestinal Worms we have forms which are cylindrical, and which present no extreme di- visions in the body (Plate XXXII, fig. C). We have others which are also cylindrical, (Pen- tastoma, Plate XXXII, figs. A, B) but in which we have transverse ridges. There are very numerous forms of the kind, which are flattened as the Tapeworm. We have others in which the differ ent parts of the body (Plate XXXI, fig. C,) differ widely—the Cysticercus. There are others in which the articulations are still more distinct, and there are again others (Plate XXVI, fig. E) in which the articulations are scarcely distinct at all, but which constitute really compound animals, as there are always two united together—Diplozoon. There are again others, which are flat. (Distoma, Plate XXVI, figs. A, B, C, D) and entirely unartic- ulated, unless we should consider as articulations those folds on the margin, which can scarcely be considered so; but owing to the arrangement of their parts, particularly that of their nervous sys- tem, we find that they must be referred to the class of Worms. Indeed although these animals have been placed in a special class, owing to the fact that they are Parasites they cannot be grouped to- gether with all other Intestinal Worms, nor form a class by themselves. They have little in common w;t!i other Parasites, but this mode of existence. In fact, Intestinal Worms constitute various types, of which the main common trait of character is to live upon other animals, rather than to resemble each other in their structure. But between Planaria (Plate XXX, fig. B) there is the rrfsst remarkable affinity. This is a Distoma. .(Plate XXVI, figs. C D) an internal Parasite, and we find that every thing agrees in the structure with Planaria (Plate XXX, fig. B). There is an alimentary canal, first a simple tube, which divides afterwards into two, and from which arise innumerable branches rami- fying in the substance of the animal. The same structure exists in Planaria, an animal which has been, referred to another class, but the resemblance is so great that it is now no longer possible to separate them; and very recently, Mr. Blanchard has proposed t to each ring; but in Terebella, which is the perfect state of the same animal (Plate XXVIII, fig. C) they are reduced to complicated gills behind the head. The vessels of the anterior gills, which occur in the anterior part of the body are indeed only modifications of these vascular threads. In the young animal (Plate XXVIII, fig. B), which has been described as a pe- culiar animal, under the name of Cirrhatulus, we have the threads all along the body, and the pos- terior threads, gradually disappear first, and the anterior ones are branched and transformed into gills; and in the beginning there are vascular threads, one to each ring. Let me now add another fact referring to this animal, that this Cirrhatulus, when young, as it is represented here (Plate XXTIII, fig. B> is pfcosphev rescent. The adult, which has been described as a Tersebella, is also phosphorescent. But in the last, phosphorescence is only noticed in the long threads,butin Cirrhatulus it is noticed all along the body. On close examination I have satisfied my- self that the blood vessels are the phorphorescent apparatus. Some such threads separated from the body when acsed upon by alcohol, or some other strong reagent, would shrow oat faint light when no other part of the animal would emit it. So that we have here an example of phos- phorescence in a position of the body different from another which we have mentioned before — This phosphorescence proceeds from the blood vessels. We have had an example from the ner- vous system. I may quote others; for instance, some Insects in which the respiratory organs, those Tracheal organs, those aerial sacs, will emit light; and the facts are such that we perceive a connection between coloration and phosphoresence and sight, as there is between electricity, heat and light. The physical phenomena are parallel to the phenomena in the animal kingdom, only it is more difficult to show their connection ; but I hope to show that there are at least among the Mollusca, some types in which it may be demonstrated that such a connection exists. [Plate XXXV—Caterpillar.! Lei me acid one more remark, that the Caterpil- lar, with all its appendages, (Plate XXXV) should be compared with the Worms. What are the di- versified hairs which are observed upon so many Caterpillars 1 They have been usually considered as hairs; but they are connected with the organs of toeomotion and respiration, as in the Annulata. We should, therefore, institute upon the Caterpil- lar a regular comparison, to ascertain whether they are not in some respects analogous to the various appendages of the Worms. This comparison ] have not instituted. It remains to be done ; but I • cannot help thinking, on noticing the close resem- blance there is between the diversified aspect of Caterpillars and Worms, that in their analogies there will be also a type discovered, as it has been noticed in the appendages of Worms; and thaa Caterpillars will only be another modification more, of one and the same type. [Plate XXXVI — Symbolical Formula of Ar- ticulata 1 LECTURES ON EMBRYOLOGY. 79 Saving introduced in one of my preceding lec- tures symbolical formulas for the three classes of Radiate animals, I deem it useful to do the same for the Articulata. Thus the symbol of the whole department will be an Omega (Plate XXXVI. fig. A) representing the curious mode of formation of CLe embryo at the inferior part of the vitellus, of which *fee two sides arise in order to envelope the vitellus. For the class of Worms we will have the same figure slightly opened at the summit, (Fig. B.| Fig. C, an Omega with a transverse bar, will repre- sent the class of Crustacea, where two regions are already distinct. Finally, Fig. D, with two trans- verse bars, for the class of Insects, m which the body "is divided in three regions. LECTURE X. When tracing the first formation and the growth lof animals, there is one point, which never should be lost sight of. It is, that at various periods of this growth, the substance of which the animal consists gradually changes. We have seen that in the beginning the germ ■consists of simple cells, derived from a modification of the yolk. Such is the first condition of ail germs. Now, from this starting-point we may ar- rive at animals so complicated as Man. In other animals, throughout the series of the animal kingdom, in which the most complicated atructures are observed—m which structures very ■distinct are successively formed,—fiesh, blood, Qerves, skin, hairs, scales, and all possible struc- tures so different as scarcely to be compared— how are these formed ? Are they new things in- troduced during the growth of toe germ—or are they only modifications, simple changes #f one and the same fundamental element, modifications of the ceRular tissue whieh characterized the germ when forming? This is a question wfcich car, be answered by facts which have been entirely investigated by one gentleman, a young physiologist of Germany, Pro- fessor Schwann. Ten years ago be began to exam- 'ine the subject of animal tissues, and up to that time it was believed that animals and plants differ- ed widely,—that their substance had nothing simi- lar, —that cells existed only in plants. Such was the condition of things in 18SS, When Schwann, taking up the beautiful investigations which Schleidenfead fast published upon the structure and growth of vegetable cells, came to the conclu- sion that animal tissues consisted equally of cells, and that whatever may be the complication of this substance in the animal—whatever may be the ex- ternal form of the various parts in the animal tis- sues—they all originate from cells, and are, after all, only modified cells. rv ibis absolute form, perhaps the results «f Schwann will have to be somewhat modified, hut in the main all subsequent investigations have only gone to confirm his unexpected result, and at present there is no student in Anatomy, who has not seen these cells of animal tissues, who is not able to find them out, even with microscopes of a very inferior quality. But it required the sagacity of the able and persevering investigator whose name I have mentioned, to start such an investi- gation—to go through with it—to give it, finished, to the world, and then to remain silent for ten years through all the attacks he has had to undergo. Since Schwann published the volume containing the results of his investigations, he has not been heard in the debates which are still going on upon this subject, it is a remarkable instance of con- fidence in his theory, and of a desire not to inter- fere with that which contradictory investiga- tions might bring about. Still it is known by his friends that he is pressing on, and preparing new investigations, which may lead to as important re- sults as his preceding labors. His efforts now go to ascertain how these cells are combined to form individuals of different kinds. Indeed, he has undertaken nothing less than to in* vestigate, if possible, the principle which combines those cells into individual cells,—to ascertain the naturec-f that power which we call vital power,—to find oat what kind of influence it is which consti- tutes individual, independent and progressive be- ings. I have delayed introducrng this subject up to the present -evening, because there is no class in which the cellular structure of animal tissues can be so fully and easily ilmstrated, as among Mollusca,— In their tissue when fall grown, in their egg when forming, the cellular structure is perfectly plain and easily ascertained. To what important results for Physiology the final investigations on this subject will lead, can scarcely be foretold now. For since it has bees PROF. AGASSIZ'S ascettained that the animal tissues are, in their fundamental structure, identical with the vegetable tissuesrwe may expect that botanical investiga- tion may tbiow as much light upon the animal kingdom,-as the study of animals may throw upon the vegetable kingdom. Easy ae it has been to study the structure of vegetable tissues, so difficult has it been to ascer- tain their functions—to ascertain the working of the various organs in plants The most different and contradictory opinions are entertained upon vegetable functions, upon the circulation of tbeir sap, upon their respiration, and the action of res piraiion upon eheir fluids. On ihe contrary, in animal structures the fnBC- tions are easily traced*. The combined action of various functions upon each other, ean be easily ascertained, It was the structure—the intimate structure—which it wasdifficult to investigate. A'nd now, by relerring the result frora one kingdom to the other, it is to be hoped that much more rapid progress will be obtained than before. One unexpected result has already been ascer- tained—namely, that eells are properly the organs of living beings; that all functions are influ- enced by life, by the independent life of isolated cells. It ta not the stomach, as a whole, which di gests; digestion is influenced by the cells which line the internal surface of the stomach. The life of individual cells may be coropaied to the action of several large organs combined into one system, as a whole. Bow much independence there is reaHy in the life of individual cells, can no where be better shown than ia some of the germs sf Moiluslts. Let me for a moment illustrate the various figures which are represented in Plate X^L. They show the changes which a MoWusk may andergo; a species of Eolis, a naked M'ollusk, found in Boston harbor, of which there is a figure in Plate XLIL Sg. € Several speeies of these Mollusks occur in Bostsn harbor, and can at any time be obtained for investigation. Several eggs which contain a single yoiX, are first noticed {Plate XL}, and hi the same plate are represented all the changes whieh tbe yolk undergoes in the process of dividing, up to- the period when the whole mass of yolk is transformed rata-in-auraer- able cells, as represented hers The divisions of these masses are not always so regular as" they have been described. In this Eolis, it does not constantly take plaeeby a reg- ular division into two halves. Ton see thnt tile two halves are more or less different in their size; sometimes the division rakes place into three spheres, two of which strt smaller than the othtr, and not even equal among ihemselves. In others, there are three equal spheres ;in others, foar equal spheres-t in others are fear less equal; in others are five almost equal -r and still in others, five, all sf which are small. Many irregularities »c«ur, Tfhere is no- invariable r»J* I Plate XL—Changes of tbe toevo v.ot.iz j You may see iu some othtr figures of the same, (Plate XL) that the process of dividing the yolk is very regular, there being first two, then four equal1 divisions, out of which may arise on one sid« fear other large spheres, and »n the «pposvt«- LECTURES ON EMBRYOLOGY. 81 Bide four smaller ones. We have still in another less regularity. Four less spheres are formed, and between them two large ones, and two very small ones; and so on, by multiplying the divisions, we arrive finally at the state of the yolk, when it is composed of a mass consisting of many yolk cells, in each of which there is a clear sphere, as there is one forming in each division when the process of dividing the yolk has only divided the mass into fewer spheres. About the time when the whole mass is reduced into small cells, there are vibrating Cilia coming out from the surface of some of these eggs (Plate XL, figs. A, B, C, D.) But the most curious phenomonon which takes place is this: that the whole yolk does not con- stantly go on to form one single individual. But there may be instances when the mass of yolk which has been subdivided into cells, is itself divi- ded into two, or three or more masses, which grow independently, several individual animals arising from one yolk;— several individual animalBaris- ing from one mass of yolk, which thus divides.— And in this process of the division of a whole mass into several individuals, there are isolated cells, which are separated from the main mass, and continue to live and to rotate by the agency of their vibratile Cilia with the main mass, And in such a case we have the wonderful sight of two or more germs, having been derived from, the di- vision of one unique mass of yolk, constituting two or three, or more individuals, each moving for Itself and rotating with the others in one yolk membrane, and isolated cells which also rotate be- tween. So that individual loose cells maintain for a time a separate life, and continue to live during the whole period of growth of the larger animals within the egg membrane; and those isolated, scat- tered cells die only when the larger germs, which will grow into perfect animals, have been hatched, or pressed out from the vitelline membrane. Nothing could show more distinctly that there is independence of life in the cell than the fact of this isolation. But what the combining power is between those cells which grow and form individ- ual animals, can scarcely be understood under such conditions. Whence the action of the vi- tal principle which keeps the cells together, oiigi- nates, escapes our intelligence. Indeed, nothing is more astonishing than to see that under slight pressure, such a germ may be resolved into loose cells, whose Cilia will continue for a short time to vibrate,in the same manner as a nebular mass seen through a powerful telescope may be resolved into individual stars, which nevertheless form a pecu- liar cluster of isolated bodies; similar to the cells with individual life, which constitute, as it were, Bimilar clusters. And when they have gone beyond this period of life, then they have undergone a more intimate connection, which prevents their dis- solving again ; and then they go on constituting a new being. Then during the further changes, by which they now assume the form of the parent animal, there are constantly isolated cells cast from the main body, which revolve for a short time, and then die. This process, which is exemplified here In the early condition of life, and under a simple condition of structure, is well known to take place in many animals, which cast their skin repeatedly during life, as the caterpillar; or Mollusks, which cast their external coating under the form of mu- cus; or other animals, which cast their hairs; or in our own body when the epidermis is cast and oth- er cells are formed to take the place of those which fall off in the form of small scales. So that you see the remarkable phenomenon of the isolated cells of Eolis, is only what we have on a still great- er scale in higher animals, where millions and mil- lions of cells are constantly cast from the surface of full grown individuals. These cells consist permanently and uniformly of an external envelope, a thin membrane containing a fluid, within which there is another vesicle called the nucleus, and in the centre of which.there is still another called the nucleolus.so that a perfect cell in its perfect condition is a sphere enclosing two other spheres, the innermost one being the smallest, ap- pearing like a granule. In such cells as are represen- ted in Plate XXXVII, we have figures with which we have been familiar from other illustrations. A cell in its perfect condition has the same structure as ah egg in its primitive formation. Here we ar- rive at a most unexpected, but universal, uniform structure, not only of cells, but of the primitive substance of wh'ch new individuals are to be form- ed. What we call eggs in their simple condition, are cells of a peculiar structure, formed in a pecu- liar part of the body, destined to undergo peculiar modifications, by which the body is not enlarged, by which no particular function is performed, but by which a new individual is formed. So that in every point of view we find uuity in the structure of animals, even in the structure, compared with the mode of re-production; the cells of which the tissues consist being identical in structure with the eggs by which new individuals are produced. There is a question which may be asked, and to which I hope to give at least a partial answer. How are these cells formed? and how are these eggs formed? We have examined the mode of formation of the germs. Let us now examine the mode of formation of the eggs. I have been fortunate enough to trace them through all their phases of formation in Mollusks, and I think there has not been a link in their trans- formation which has escaped my attention. So that the whole process of their multiplication has been directly observed. Tracing the formation of eggs will be tracing the formation of cells, the mo- ment it is understood that cells and eggs have the same structure. When examining very young ova- ries—for we must not take the egg when laid—we must not take them when formed within the ovary —we must not take even a full grown ovary—but 82 PROF. AGASSIZ'S take the ovary when forming and examine what is produced. There we observe that the ovary consists of pouches (Plate XXXVIII, figure A)—folds of [Plate XXX VIII—Ovisacs and Eggs of As- CIl>fA.] [Plate XXXVII —Formation of Germs.) membranes, in each of which bottle-shaped pouch- es (fig. B ) there are masses of eggs and other sub- tances—granulated substances—and complete eggs in the larger ones. You may perhaps distinguish from the distance that in such a pouch (fig. A ) which is circumscribed by a membrane, there is a mass of little granules and a number of eggs, each having a vitelline membrane with its germinative vesicle and its germinative dot. The smaller of these pouches contain the same elements. These smaller ones will contain fewer eggs. The still smaller one will contain also eggs, but they are not so well defined. And we may find some pouch- es in which there are no distinct eggs, but a bag full of uniform, clear liquid. Here is the starting point. And if we examine under a very high power what is going on in these pouches, we may observe all the changes which are represented (Plate XXXVII) in these various fig- ures. First a little bag is observed, but perfectly transparent and homogeneous. Others may grow larger, but still contain transparent homogeneous fluid. All these figures are represented under the same magnifying power. Then we may find one in which the membrane surrounding the liquid di- vides. This process of dividing is observed in the yolks when fully grown, giving rise to the embry- onic cells; here it takes place to form numerous eggs, giving first rise to two continuous vesicles, one larger than the other, which may grow to an equal or to an unequal size—the one dilating, the .other growing less, may give rise to two'half vesi- cles. Next, they may grow larger. Next, we ob- serve that granules are formed. Here we have the first element of heterogeneous substance. Granules are formed within. How such changes are brought about is not understood. It is a mystery in the subject of our investigation. But that it takes place can be easily seen. Now, these bags being full, no longer of a uniform liquid, but of a granulated liquid, will un- dergo the same change. They will divide into two sacs, which will grow equally or will remain unequal, and we shall have the process of separa- tion as observed here. But as soon as granules have become numerous, there is a condensation taking place in some point. These granules ag- glomerate in some point .without having a mem- brane about them. There is simply a dense con- densation of granules in one point. And this con- densation will grow larger, so that the condensed sphere within the granulated liquid will successive- ly be larger and larger ; or by the side of the large one there will be several small spheres developed, growing at some distance from them, and remain- ing isolated.. And perhaps some two such spheres will begin to separate, or a separation of the part which contains only clear granules from the part in which a condensation has taken place, will occur in this way, and then those spheres with two cen- 12 35 LECTURES ON EMBRYOLOGY. R3 tres of concentrated mass will begin to separate, as we have here (Plate XXXVII, fig. 2) where we have two distinct spheres, with a concentrated mass in each At this period, each of these concentrated masses is without an envelope. And now there will be an envelope formed around it. And here it will grow into a hollow vesicle; and as soon as this last process has taken place, we have a free egg. Around the spheres of condensed gran- ules a membrane is formed, and some one or sev- eral of the granules within growing larger, give rise to a perfect egg. And s"o we see in the larger and still larger, those concentrated collections take place and go on developing as we have them here (Plate XXXVIII, fig. B.) with a mass of con- densed yolk,swimming in a granulated liquid. And then the eggs escape from these pouches, and are laid, under their normal form. Then begins the series of modifications and repeated divisions and subdivisions which give rise to the formation of a germ to form a new individual. Now, the changes of these eggs illustrate the same time the formation of cells. They are multiplied by the division of vesicles containing a simple liquid. Condensation takes place within and around this collection of granules, and a membrane is produced. Then will appear again some granules growing within, which will be the nucleoli. It can now no longer be doubted, that the pro- cess of formation of eggs and the process of formation of cells, are identical, as it was under- stood that eggs and cells.in their perfect formation, were similar organizations. I would now proceed to illustrate the further changes of the germ of Mollusks—to show how the young of the Mollusks are developed—how they successively assume the form of the perfect ani- mal, and how their various organs are developed. Here is a diagram (Pjate XXXIX), which gives a general view of the rapid successive changes which the eggs of Cuttle Fishes undergo, in which the germ is formed around the yolk (Fig. B). After some changes, the outline of the young animal is formed (Fig. E), 'and after some other changes (Fig. F), it begins to resemble the full grown ani- mal (Fig. G); and before the animal is hatched, we see it really does resemble the Cuttle Fish. (Plate XXXVI, fig- A). You see (Plate XXXIX, fig. G.) the body.the eyes the tentacles. &c. But in order to show that all Mollusks have the same mode of formation, not- withstanding their apparent diversity, I must be- gin by showing you that the perfect animals them- selves are constructed upon the same plan. And this is no easy task. There is no group of the ani- mal kingdom which has been more studied, and no one which is less understood than that of the Mol- lusca in their morphology. I do not say that there is no group in which species are less known. On the contrary, few departments of the animal kingdom have been more extensively studied in the details—in the distinction of genera and species. [Plate XXXIX—Embryos of the Cdttle Fi«-h Thousands of them have been well figureu an>l de- scribed. But the correspondence of their parts, from one class to another—the analogy of the dif- ferent organs in their various positions—this is what is not understood in this class of animals. That all Mollusks agree in the softness of their bodies, is well known. And this character has been constantly insisted upon as the distinguish- ing character of Mollusca—a soft, contractile body without articulation. This is the general charac- ter assigned to the type of Mollusca. And in ad- dition to their character, derived from the external appearance, is usually added the fact that they have a nervous system, consisting of a circular ring around the entrance of the alimentary canal, with a swelling above and below, forming a single ring without a chain of repeated swellings, as is observed among the Articulata. But that Mollusks agree beyond this, in their structure, is so little understood, that in our descriptions,we find groups contrasted In which it is said that the gills are 84 PROF. AGASSIZ'S [Plate XXXVI—Clttle Fish.J h\*4 upon llic o.ti k ; oilieis in Mtiicti it la saui that the gills are upon the sides or on the lower side of the animal; and others in which the eyes are said to be in an entirely different position from wwhat is observed in others. Indeed, no analogy has been, nor can properly be traced between these animals. I have, however, taken pains to trace analogy, and if I am not mistaken, have succeeded in making it out. But if I shall equally succeed in satisfying you, is another question, which you may decide after my illustrations have been made. Let us begin with an animal well known in its form and structure. Let us take the Oyster or the Scallop If we lift one shell, we see that it is lined inside with a membrane called the mantle. The two valves of the Scallop (Plate XLIV, fig. A) as you see them drawn here on a large scale, are both lined with the mantle On opening these two valves, you see the mantle on both sides. The membrane, as it lines the valve of the right side, is seen in Fig. B. The membrane which lines the opposite valve, which is removed, and which cov- ers the interna! organs, is removed with the shell. These two membranes lining the shells hang on the two sides of the animal. So that the mass of organs, the gills, the muscles,the liver, and alimen- tary canal—the whole structure is contained, as it were, between those two folds—those two mem- branes—as the contents of a sac within its walls. Or I may compare the shell to the coat, the lining membrane to the waistcoat, and the organs to the body within. [Plate XLIV—Pecten—Scallop-shell J The position of the eyes is very remarkab;e in this animal. There is a series of eyes (Plate XLIV, fig. B) all around the margin of the mantle —about forty or fifty, or more, in number. And you see that they occur upon both sides, so that it is like a row of buttons along the coat, forming here two rows of eyes, [laughter]; and this posi- tion is so extraordinary that we may not expect to find any analogy with the Cuttle Fishes, (Plate XXXVI. fig. A), where we have two large eyes up- on the sides of the head, or with Strom bus, as we have in Plate XXXIII, where we have two large eyes, upon peduncles, on the two sides of thepro- [Plate XXXIU-Strombus.I LECTURES ON EMBRYOLOGY 85 bescis, which comes out from the mouth. We might not expect to find these eves about the head in any way analogous to the large number of eyes which surround the margin of the mantle. Nevertheless, if I have understood the structure of Mollusca, I shall show that these eyes are all the same as those of the Oyster, the same as those of the Cuttle Fi. TU wa v„„« LECTURES ON EMBRYOLOGY. 87 therefore, here, series of eyes all around the mantle. We have even the series nearly as completely de- veloped, as in the Pecten, and we have a fully de- veloped eye on the anterior part of the head, on the side of which, there is one larger tentacle ob- served ; making the analogy perfect. But let the lateral rudimentary eyes disappear and the anterior pair remain, and we have the ordinary condition of Gasteropoda:; so that the question whether there is anysimilarity between the Acephala and other Mollusks, must be answered by the assertion that the analogy is as complete as can ever be expected between animals of the same Rreat department, but belonging to different classes. Indeed, in tracing the differences between the man- tle of Margarita, 1 Plate XLIIL) and that of the Acephala, we notice the anterior part of the mantle has larger fringes corresponding to the region where those larger eyes occur. So that we have an uninterrupted series from those in which there are eyes all around, gradually to those which have eyes only a part of the way round, and to those which have only two eyes. Tracing, however, this structure further down, we come from Pecten to shells, as in Mya, where there are no eyes at all. But even in these, there are colored specks at the open- ings of the mantle. So that we have a natural ap- paratus with compoundeyes, with perfect lenses,in one order of Mollusca, as they exist in vertebrata, down to these which have eyes with a rudimentary crystalline lens, and still further down to those specks which can enable the animal hardly,if at all, to distinguish between light and darkness. Here we have a new species of a so-called soft shelled Clam, {Ascidia} (Plate XLI,) in which the animal is included within a sac, and leaving only two openings at one end. Now on the ends of these openings we have in this—anew species, LECTU LECTURE XI, In every type of the animal kingdom, there have been some forms observed which have perplexed Naturalists, and whose natural positions have not been ascertained until after extensive investiga- tions. You remember with what difficulties we struggled when examining the natural circumscrip- tion of the type of Radiata ^ how, many animals, which had been considered as Polypi, had to he excluded from that class, as it must be circum- scribed by the observations of modern investiga- tors. Among Articulata, we felt the same difficul- [Plate XLI—Ascidia or Soft-shelled ClamJ Ascidia scutella—which I have observed recently in New Bedford—colored dots. What are they ? The last indication of the lowest condition of eyes en the margin of those tubes, through which water is introduced into the body. And through these, and through the open tubes of Clams, we pass gradually to those more complicated organs, as they are seen in the higher species.with a pair of eyes. From those in which we have eyes, to those in which we have only colored dots, we have grad- ual steps. And in this way from the most regular Cephalo- poda (Plate XXXVI, fig. A.) down to the Ace- phala, (Plates XXXV, XLIV and XLI) we have the multiplication of these organs, tending to transform well-defined organs into single colored specks. In my next lecture I shall say a few words more upon the structure of Mollusca, and then proceeC to illustrate their embryonic growth. ties, owing to the peculiar structure of many par* asitic Worms, of many parasitic Crustacea, which, when full grown, differ so widely from their em- bryonic condition.that they cannot be arranged with them, unless the whole history of their metamor- phoses be ascertained by embryonic investigations. The same difficulty occurs with Mollusks. If we had only to deal with animals with bivalve shells, with the Snail-like Gasteropoda, or with the Cuttle-fishes, as I showed in my last lecture, the general structure could be traced in their outlines, 83 PROF. AGASSIZ'S and there would be no doubt left as to the final eircumscription of that group. But there are animals which mast be referred to the type of Mollusks, according to our present knowledge of their structure, which differ so wide- ly in their appearance from Mollusks, that, at first, when mentioned, this combination seems utterly unnatural and anfounded ; and indeed, leaving the impression as if there could be no foundation for a natural system, if such combinations were to be considered as natural. Nevertheless, I think that the association of some animals which I am about to illustrate, will be found to rest on real affinity; and that the external differences hi form will have no influence npon the impression which such a combination will leave [See Plate XXXIX. page S3 ] We have here in Plate XXXIX, and in several oth- er diagrams [which the Professor exhibited to the audience J Polype-like animals, re-enabling Polypi very much by their stems, with cells in wbieh there are liviBg animals extending and contracting in a manner similar to Polypi, with tentacles around their mouths, which aet in a manner resem- bling Polypi still more than the stem in whieh they are included. And these animals do not belong to the tvpe of Polypi; they are true Mollushs. The discovery of their internal structure was made almost. simultaneously by Ehrenhrg, by Milne-Ed- wards, and bv Mr. Thompson, of Cork,so that their relation to Mollusks is now known to be very close They have a relation to the radiated type of Polypi by the fringes around the mouth.-— But the arrangement of their whole system is truly bilateral. This figure (Plate XXXIX. fig. C> represents the alimentary canal, which differs very much from the Radiata, in being curved upon- itself, in having distinct openings, a large sac which represents the stomach, and a structure which comes verv near that of some animals whieh have never been sep- arated from MoHask?. If we were only to consid- er those, perhaps the resemblance to Mollusks might have escaped observation. But let me now trace further than I did before the analogies which exist between Mollusks. I compared the Gasteropoda with the Acephala and the Cephalopoda; * showed that there was one type in the bivalves, in the univalves and in Cephalopoda. But between the Ascidia) (Plate XLVII), and the Clams, (Plate XXXV), there are only slight differences. Suppose the shell of the Clam (Plate XXXV) to disappear, the mantle to be almost entirely removed, the respiratory tube to be shortened, and the two openings to be somewhat remote, and we shall have such an an- imal as is represented in Plate XLVII, fig. H, en- closed in a sac with two openings, which are not the openings of the alimentary canal, but are the openings which lead into a cavity in which all the organs are contained. [See Plate XXXV, page 7S.J [Plate XLVII-Ascibue J And now going further, we may have all possi- ble modifications of this type when it is contracted and when the peduncle is attached. Plate XLVII, fig. A, represents a fixed Ascidia, the peduBcle be- ing only a prolongation of the sac-like envelope. Here we have two openings of the sac, correspond- ing to the two openings of the clamshell. Beyond this type, we may have one in which several indi- viduals are united by tbeir base. And then, from singie animals, we pass to compound animals combined by their attachment on one spot, (Plate XLVII, fig. F) or by a gelatinous envelope which keeps the eggs together, (Fig. B), aud constitutes compound animals. The internal structure of these Ascidise, (Plate XLVII, fig C ), Is so like that of Clams, that there is no difficulty about their analogy. Now, one step farther, and suppose that the gelatinous en- velope which unites these individuals secretes calcareous substance. Suppose further, that each individual is much smaller, and in addition, that one extremity, instead of presenting fringes1 at its opening, is surrounded by threads ; then you have the structure of the Bryozoa. (in Plate XLVII 1)-, with a calcareous stem, with a sym- metrical alimentary canal, but with serrated tenta- cles round its anterior aperture, constituting a pe- culiar type—the Bryozoa. And that they are *w> LECTURES ON EMBRYOLOGY. 89 these Bryozoa, which have been investigated by Professor Van Beneden, (Fig. A). The bud-like egg which arises from the main cavity does not produce a terminal germ, from the lower centre of which the main cavity proceeds, but produces (Figs. D, G) a division of this yolk like mass, un- dergoing all the processes of division which we have elsewhere observed, and finally assuming an elongated form. From the beginning it exhibits the peculiar character of Mollusks, which distin- guishes them from Radiata. Their bilateral form, on the longitudinal axis, is observed in these germs. And thus, going on further, the margin becomes serrated, (see Fig. E), the internal cavity growing deeper and deeper, introducing the whole mass of yolk within, (Figs. H, K) with appendages above. These appendages will soon open, and you will have (Plate XLVIII, fig. C) a large alimen- tary canal, with a central cavity placed in a dis- tinct cavity of the body, with tentacles round the opening; so that this structure is distinct from that of the Radiata. But I must dissent from the conclusions which Professor Van Beneden has deduced from his ob- servations. From the manner in which the yolk is placed in the interior of the alimentary canal, be concluded that there is no difference between the Radiata and the Mollusca in their embryonic erowtb, as the yolk is formed around the cavity, and as the yolk is introduced from the lower side in both. But he overlooks that in Radiata the centre of development is really the centre of the mass, and that the further growth takes place in all directions simultaneously, by a uniform, all- sided development; whilst in Mollusks there is from the earliest period this bilateral and longitu- dinal axis. We might just as well say that the Vertebrated Animals do not differ from the Radi- ata, because in the former the yolk also is introduced from the lower side into the animal. But'M •'0$m '' -yMkLi , t''i- »'",.• v. - v/.i.i-i