THE DEVELOPMENT OF FLOUNDERS, 
BY ALEXANDER AGASSIZ. 
fpHE manner in which the eyes of a flounder become placed on 
-*• one side of tlie body has formed a fertile subject for theories. 
I do not at present propose to discuss the explanations given to 
account for the facts, but merely to state the results of observa- 
tions made while studying the development of a number of spe- 
cies of flounders common to our coasts. In the case of five spe- 
cies the passage of the eye from one side to the other is not, as 
urged by Malm, a simple tendency of the eye of the “ blind side ” 
(the side upon which the flounder lies) to turn towards the light 
and thus carry the surrounding parts of the head with it. The 
eye placed on the blind side actually travels from its original 
place (symmetrical with the eye of the opposite side) frontwards 
and upwards on the blind side, resorbing the tissues in its way, 
new tissues forming behind ; there follows this movement of trans- 
lation a certain amount of torsion of the whole of the frontal part 
of the head, which commences only after the eye of the blind 
side has nearly reached the upper edge of that side, quite a 
distance in advance of its primitive position. This torsion of 
course takes place most readily, occurring as it does during a 
stage when the whole bony fabric of the skull is still cartilagi- 
nous, and it is the torsion which ends in bringing the eye to the 
opposite side. In four of these species of flounders the dorsal 
fin did not at that young stage extend to the posterior edge of the 
orbit of the eye coming from the blind side. 
In another species, after the eye had thus by the same process 
of translation and of torsion been brought from the one side to 
the other, the dorsal fin gradually extended beyond the ante- 
rior edge of the orbit of this eye. This young flounder thus soon 
presented a stage in which the eye from the blind side appeared 
to have passed through the head between the frontal bone and 
the base of the anterior rays of the dorsal fin. As 1 had, however, 
followed the whole development in living specimens, I knew from 
actual observation that the mode of transfer of the right eye had 
been identical with that of the preceding species.v These obser- The Development of Flounders. 
2 
vations thus far 'confirm in the main Malm’s explanation of the 
development of young symmetrical flounders into the well-known 
older stages. To my great astonishment, therefore, I captured 
one day a number of flounders (about an inch in length) closely 
allied to the Plagusice of Steenstrup, the so-called Bascania of 
Schiodte ; they were so perfectly transparent that they seemed 
the merest film on the bottom of the glass vessel in which they 
were kept. They were still entirely symmetrical, the eyes well 
removed from the snout, with a dorsal fin extending almost 
to the nostril, far in advance of the anterior edge of the orbits 
of the eyes. They were of course at once set down (from their 
size) as belonging to a species of flounder in which the eyes 
probably remained always symmetrical, and I prepared to watch 
its future development. It was therefore with considerable in- 
terest that I noticed, after a few days, that one eye, the right, 
moved its place somewhat towards the upper part of the body, 
so that when the young fish was laid on its side, the upper half 
of the right eye could be plainly seen, through the perfectly trans- 
parent body, to project above the left eye. The right eye (as is 
the case with the eyes of all flounders), being capable of very ex- 
tensive vertical movements, through an arc of nearly 180°, could 
thus readily turn to look through the body, above the left eye, and 
see what was passing on the left side, the right eye being of course 
useless on its own side as long as the fish lay on its side. I may 
mention here that this young flounder, until long after the right eye 
came out on the left side, continued frequently to swim vertically, 
and that for a considerable length of time. This slight upward 
tendency of the right eye was continued in connection with a 
motion of translation towards the anterior part of the head till 
the eye, when seen through the body from the left side, was en- 
tirely clear of the left eye, and was thus placed somewhat in ad- 
vance and above it, but still entirety in the rear of the base of the 
dorsal fin extending to the end of the snout. What was my as- 
tonishment on the following day, on turning over the young 
flounder on its left side, to find that the right eye had actually 
sunk into the tissues of the head, penetrating into the space be- 
tween the base of the dorsal fin and the frontal bone, to such an 
extent that the tissues adjoining the orbit had slowly closed over 
a part of the eye, leaving only a small elliptical opening, smaller 
than the pupil, through which the right eye could look when the 
fish was swimming vertically. While the young flounder lay on 
its side, the right eye was constantly used in looking through the The Development of Flounders. 
3 
body, and could evidently see extremely well all that took place 
on the left side. On the following day the eye had pushed its 
way still farther through, so that a small opening now appeared 
opposite it, on the left side, through which the right eye could 
now see directly, the original opening on the right side being 
almost entirely closed. Soon after, this new opening on the left 
increased gradually in size, the right eye pushing its way more 
and more to the surface and finally looking outward on the left 
side with as much freedom as the eye originally on the left; the 
opening of the right side having permanently closed. I have thus 
in one and the same specimen been able to follow the passage of 
the eye from the right side to the left through the integuments 
of the head, between the base of the dorsal fin and tire frontal 
bone. 
This observation leads to somewhat different conclusions from 
those of Steenstrnp, who thought he could prove (from an examina- 
tion of alcoholic specimens) that the eye from the right side passed 
under the frontal bone. This is evidently not the case here, the 
eye passing round it, there being only a very slight torsion of the 
frontal in this young stage. Although at first glance this appears 
so radically a different method of transfer of the eye from the one 
described above, yet if the dorsal fin had not extended beyond 
the posterior edge of the right orbit the process would have been 
the same, as is readily seen. I hope soon to give full details, with 
illustrations, of the process of transfer of the eye in its different 
stages, in a paper I am preparing on the young stages of a few 
of our bony marine fishes. 
But while I have thus been able to trace step by step in living 
specimens the transfer of the eye from one side to the other, I 
can give no explanation of the cause which compels flounders to 
lie on their side. The explanations usually given are not satis- 
factory. For the great depth of the body, the position of the dor- 
sal and of the ventral fins, the undulating mode of swimming, — 
all these are so many causes specially adapted to enable them to 
swim in a vertical position. In fact, they always swim vertically 
in young stages, when their capacity for vertical support is in- 
finitely less than when they commence to lie on one side. 
The rapidity with which the young flounders adapt themselves 
to the color of-the ground upon which they are placed is marvel- 
ous. In one of the species the red, yellow, and black pigment 
cells were brought into the proper combination and prominence 
with such rapidity that it seemed hardly credible that the same 4 
TEe Development of Flounders. 
fish could assume such different hues in so short a time. The size 
and number of these pigment cells, however, readily account for 
all this. 
The young of this transparent flounder do not invariably lie 
down on the right side; it seems almost a matter of chance which 
side they choose. Out of fifteen specimens eight lay down on 
the left and all died without being able to accomplish any part of 
the transfer of the left eye to the right side, although they lived 
quite long enough for the other seven, which turned down on the 
right side, to accomplish the transfer, or nearly so. This in- 
capacity may account for the rarity of sinistral forms of floun- 
ders, and vice versa. In the other species mentioned above, all 
the young 1 have had occasion to keep alive turned down on 
the proper side for a successful transfer of the eye. I may also 
mention here that at a certain stage of growth of our Ctenolabrus 
the young show a very decided tendency to lean on the right side. 
This is even seen in fully grown specimens, in the peculiar slant- 
ing position often taken by individuals when they approach an 
obstacle.