Tracy M. Sonneborn - An Appreciation

D. L. Nanney

Provisional Department of Genetics and Development
University of Illinois
Urbana, Illinois 61801

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Two roads diverged in a wood, and I --

I took the one less traveled by,

And that has made all the difference.
--Robert Frost

The papers in this volume of Genetical Research are submitted by his students and
associates in tribute to Professor Tracy M. Sonneborn on the occasion of his seventieth
birthday. For nearly forty years, Sonneborn has trained most of the investigators in cil-
iate genetics, nearly three dozen as doctoral students, nearly two dozen as postdoctor-
als, and indirectly, ever rising numbers of scientific grandchildren. He remains the un-
challenged master, the chief repository of wisdom, a steady source of inspiration and
challenge.

Ciliate genetics, at least in its modern phase, is largely a Sonneborn construc-
tion but it was not built in a vacuum. Sonneborn was one of the doctoral students
(1928, Johns Hopkins) of Herbert Spencer Jennings, and he elaborated many of Jennings‘
values and interests. Unlike some other students of the protozoa, Jennings refused to
consider protozoa as creatures apart from other life forms, but sought through their par-
ticularities to discover principles of general application; in this sense he was a biolo-
gist rather than a protozoologist. Most of Jennings' later studies were limited to the
protozoa, but they ranged from pioneer explorations in cellular behavior and clonal aging
to the beginnings of population genetic analysis. Sonneborn likewise refused to be limi-
ted to one level of life's hierarchy or to one narrow discipline. He resembled his mentor
also in certain personal characteristics. Notable in both men is a penchant for careful,
systematic, even painstaking analysis.

But Jennings’ experimental work was hampered by his inability to control mating in
his protozoa, and Sonneborn's first major contribution to the field came in 1937 when his
discovery of mating types in Parmecium aurelia overcame that barrier. The exploitation of
controlled mating led quickly to several fundamental discoveries. He found that Parame-
cium aurelia (and indeed most ciliate "species") is a species complex composed of many

well-defined and effectively isolated gene pools. It is an interesting historical aside
that Sonneborn's discoveries permitted a new and important chapter in Jennings’ own re-
search career that continued until his death.

The "biological species" of the ciliates were first named "varieties," later "syn-
gens," and some have finally achieved full legitimization as taxonomic species. In spite
of their apparent similarities, these sibling species sometimes show large differences in
important biological attributes. Sonneborn's comparative studies of these ciliate life
histories led him into one of his most seminal but still ill-appreciated explorations --
into the poorly defined area of breeding economies. In a careful analysis of the special
adaptations of the ciliates, he showed that most of their hitherto unexplained peculiari-
ties were related to a single central issue: "Who mates with whom." The ciliates provide
an otherwise unexampled array of very similar organisms living in very similar habitats
but ranging from obligate inbreeders to compulsive maters with strangers. The ecological
and evolutionary genetic of the ciliates are only in their beginnings, but we must ex-
pect interesting developments from these organisms which have for so long accumulated
molecular variation while being constrained in narrow ecological niches. The ciliates may
provide unparalleled materials for the study of evolutionary strategies.

One of the studies for which Sonneborn is best known is that dealing with kappa,
the "killer" cytoplasmic factor. These cytoplasmic elements were for several years the
best understood of the "plasmagenes." They were considered important at first because
they provided a possible resolution to the developmental riddle: the "hereditary" differ-
ences in cell lineages with identical nuclei. The significance of plasmagenes in develop-
mental differentiation diminished with better understanding of nuclear capabilities, and
also with the recognition that kappa particles were bacteria rather than some simple
gene-like entity. Later, however, the kappa work provided a model for the study of other
cytoplasmic elements, such as chloroplasts and mitochondria, which are more general pro-
toplasmic structures. The kappa work also led to theoretical confrontations around the
issues of native and foreign cellular constituents, of norma] and infectious genetic ele-

ments. These considerations foreshadowed the discovery of episomes, and they reverberate
still in our search for the origins of organelles and the mechanisms of cancer. In more
recent years kappa has been studied by one of Sonneborn's students (and several grand-
students) as an interesting thing in its own right, as a component in an exceedingly in-
timate symbiosis. It continues to provide surprises as new interactions are exposed.

Another trait simultaneously studied by Sonneborn and his associates seemed ini-
tially to be very similar to the killer character. Differences in immobilizing antigens
of Paramecium behave in some crosses in precisely the same way as do differences between
killers and sensitives; with cytoplasmic mixing, exconjugants become alike; without cyto-
plasmic mixing exconjugants remain different. Yet the Sonneborn group showed that the
specificities being regulated were encoded by nuclear and not by cytoplasmic genes.
Sonneborn was one of the first to recognize that "cytoplasmic inheritance" is not a phe-
nomenon, but phenomena, with a diversity of mechanisms. Cytoplasmic inheritance need not
imply "self-reproducing" cytoplasmic elements. Some detailed mechanisms for the perpetua-
tion of differences in "nucleocytoplasmic states" have been discovered in procaryotes,
but the first clear demonstration of the phenomenon was provided in a eucaryote. Perhaps
the first detailed understanding of a mechanism for maintaining such states of gene ac-
tion in a eucaryote will yet come from the ciliates, as current studies in the chemistry,
genetics and physiology of serotypes move forward.

Other studies concerned with the perpetuation of cellular differences date from
the same early period and continue to the present. Sonneborn showed that mating types in
many ciliates are epigenetically determined. Through a series of clever and meticulous
experiments he demonstrated that mating type determination is achieved by a specific al-
teration in the nucleus during a critical time in its development. The capabilities of
the nucleus are restricted by its genotype; its development is influenced by the environ-
ment and, in some cases, by the cytoplasmic surroundings. In the Jater case an example of
"cytoplasmic inheritance" was exposed as a cryptic case of nuclear differentiation. When
these experiments were first reported their general significance was questioned, because

the notion of nuclear regulation was not yet popular. Mating type inheritance in ciliates
 

still provides one of the clearest examples of nuclear differentiation, and recent suc-
cesses in some ciliates in defining the chemistry of the mating substances offer encour-
agement that we may soon understand the mechanisms.

In recent years Sonneborn has devoted considerable efforts to an understanding of
the ciliate cortex and to its role in morphogenesis. These studies once again challenge
some commonly accepted verities. They demonstrate structural inertia in growing systems,
whereby differences in organization of cellular elements tend to maintain themselves in-
definitely - even when supplied with the same raw materials and grown in the same en-
vironments. The role of pre-existing structure (cytotaxis) in guiding new structures is
abundantly documented in ciliates and is certainly of some significance in all living
systems.

This brief survey does poor justice to the diversity and depth of Sonneborn's ex-
perimental work. He also studied clonal aging in ciliates and followed these interests
with explorations into human aging. He has conducted over a long period of time an heroic
campaign to induce and identify genetic markers for all the 40-plus chromosomes of P.
aurelia. He has carried out comparative studies on all the genetic species and has
brought under cultivation still other protozoans.

In addition to all this, Sonneborn has been a majorsmotive force in several areas
of ciliate biology in which he has never associated his name directly. Unlike the prac-
tice in many other traditions, Sonneborn would only accept coauthorship for work in which
he made a substantial observational or experimental contribution. Thus, most of his doc-
toral students and post-doctoral associates published their research alone, though only
we can know the countless hours of discussion and writing he invested in each of us, and
the rarity of the sentence that passed his scrutiny without modification. Primary scien-
tific contributions were sometimes transmitted through his formal classes (in which he
reached far beyond the protozoa) or through his perenially stimulating weekly seminars
(in which our analytic powers were challenged to their limits) or through his one-on-one

editorial interactions (through which we learned, if ever we could, the meaning of pre-
cision). Many ofthe foundation stones on which his students built were given ready-cut,
and never adequately acknowledged.

His protean efforts make final evaluation and summary very difficult. In prepara-
tion for this essay, I wrote to some of his former students asking guidance on this
point. Specifically I asked for what contribution Sonneborn should best be remembered in
2075. Nearly all the respondents identified Sonneborn's preeminent role in promoting the
use of ciliates in experimental biology, When individual accomplishments were mentioned,
however, they ranged very widely, from his discovery of mating types, through his studies
on cytoplasmic inheritance, to his recognition of significant roles for cellular fabrics
in cellular morphogenesis. Although numerous contributions of great diversity were re-
called, no single biological principle or grand generalization surfaced. Indeed several
respondents remarked that it is too early to give a final judgment on the significance of
Sonneborn's contributions, not merely because his work continues through his own efforts,
but because his work continues also through the work of others. As one respondent notes,
when the work he initiated on clonal aging continues to an understanding of its mechan-
ism, and when his work on circadian rhythms is resolved in an understanding of cellular
oscillations, and when his comprehensive interactional approach to cellular morphogenesis
yields a more realistic understanding of cellular organization, then we will be able to
evaluate what he should best be remembered for. To a greater extent than with most inves-
tigators, Sonneborn's contributions are seed in the ground. We cannot yet know which are
most likely to flourish and flower.

Lest one misjudge this issue, it should be noted that this situation derives from
a deliberate policy and a personal philosophy of research. Sonneborn has said on several
occasions that he would not do an experiment if he thought it likely that someone else
would do it. He jis not challenged by the race course but by the far horizon. Given a
choice of roads, he takes the one less traveled by. He is an explorer, interested in mak-
ing new beginnings, rather than an exploiter willing to mine a rich vein. The result is a

corpus of contributions, incomplete in important respects and difficult of final evalua-
tion but certain of a long half-life.

Although one may argue which of his scientific efforts are most weighty, agreement
is easily reached about his personal characteristics. I asked my sample of Sonneborn as-
sociates to identify "the chief personal characteristic which has affected your relations
with him and/or science." Although the responses came back with many diverse turns of
phrase, the consensus was unequivocal. Above all else, Tracy Sonneborn is a master tea-
cher, one who has accepted with high seriousness the master teacher's double duty to res-
pect the truth and to respect the recipient of that truth.

At a time when undergraduate instruction was often considered a burden and a hin-
drance, and when his “star" status could have given him exemption, Sonneborn cheerfully
carried far more than his required teaching load, and he carried it with ever renewed en-
thusiasm. Although he taught the beginning genetics course for many years at Indiana, it
was always a new course, with new organization, new text books, new subject matter and
new ways to communicate. The preparation of a lecture was always a serious business
worthy of hours of intense and patient preparation, even though, or perhaps thereby, the
delivery of the lecture was a dramatic and exciting happening. He could never accept from
himself less than his very best efforts, and that fact conditioned mightily his students’
response to his teaching.

The time Sonneborn spent in preparation is difficult to appreciate for one who did
not experience him. His advanced course in microbial genetics usually focused on one par-
ticular organism. He was never content to read the current literature about that organism
and to deal with recent reviews. He read, analyzed and annotated every significant paper
in the history of the subject. By this means he communicated to his students his deep
concern for the literal truth and an almost rabbinical respect for the written word which
could not have been sustained by a casual treatment of his sources. He proved his belief
in the importance of his material by the time he invested in its analysis. I once asked
him how he could afford to spend so much time on his teaching. He answered that it wasn't

difficult. He owed the university a 40-hour week for his teaching and housekeeping
efforts. So long as his second 40 hours were free for his own research activities, he had
no reason to complain. He not only did not complain, but he actively sought involvement
in teaching activities. Even near the end of his formal teaching career, he requested and
taught with enormous success a large scale popular course in the meaning of biology for
non-science students.

Sonneborn's teaching has never been limited to the classroom; indeed for his grad-
uate students and associates that manifestation was sometimes neglected, until our own
involvement in the classroom brought a delayed appreciation and understanding of his ac-
complishments there. His involvement with his research students is intense and personal,
selfless and life-long. He takes every observation of nature as a serious and important
event. He is excited by the unknown and the unexpected. He insists on an accuracy of ob-
servation, a rigor of design, and a perfection of expression which demands all a student
is capable of. He can insist on high standards of performance and sometimes can achieve
them because those standards are exemplified in his own work.

Sonneborn's magisterial relationships are not limited to a definite period and
fixed place. Most of his students never leave his tutelage entirely, and many investiga-
tors the world over have had the benefit of his careful and thoughtful attention. Commen-
ting on Sonneborn's many hours of selfless analysis and rewriting of manuscripts, one
foreign respondent comments that “His efforts always touched us to our heart." If someone
else is willing to spend that much valuable time and thought on our work, it and we can-
not be entirely lacking in importance. Sonneborn's chief gift to his students jis precise-
ly this, a constantly renewed sense of high personal worth and human dignity, a sense of
sharing in an important human endeavor.

One cannot evaluate Tracy Sonneborn without acknowledging that he is only a part
of a remarkable team. Sonneborn was able to devote extraordinary hours to his profession-
al work because his personal life was flexible and as untroubled as a mortal can hope
for. He was able to extend sympathy and understanding to his colleagues and students be-

cause of the security of his own domestic situation. We cannot honor Tracy without also
honoring Ruth, and thanking her for sustaining his career and enriching all our lives.
Tracy Sonneborn has had a fair share of high honors. He fs a member of the Nation-
al Academy of Science, the American Philosophical Society and the American Academy of
Arts and Sciences. He has served as president of the American Society of Naturalists, the
Genetics Society of America, the American Society of Zoologists and the American Insti-
tute of Biological Sciences. He is a Foreign Member of the Royal Society of London and of
the French Society of Protozoologists. He has received the Newcomb Research Award of the
AAAS and the Kimber Genetics Award of the National Academy of Sciences. He has received
coveted teaching awards at Indiana University where he has spent most of his professional
career, and he was awarded honorary D. Sc. degrees from Johns Hopkins University, his
alma mater, and from Northwestern University, and the University of Geneva. Our purpose
on this occasion is again to acknowledge achievement and to do honor by presenting sam-
ples of that work in which Sonneborn has made so profound an investment. Above all we
want to express our gratitude for having touched one who is, as one respondent notes,

"simply the most remarkable human being I've ever known."
SELECTED SONNEBORN BIBLIOGRAPHY

The following references, compiled from among 216 publications to date, have been selec-
ted as representative of the breadth and productivity of Sonneborn's career. It should be
noted that many of his important discoveries have been announced solely in abstracts.

Sonneborn, T. M. 1930. Genetic studies on Stenostomum incaudatum (nov. spec.) I. The na-
ture and origin of differences among individuals formed during vegetative reproduc-
tion. Journal of Experimental Zoology. 57:57-108.

Sonneborn, T. M. 1930. Genetic studies on Stenostomum incaudatum. II. The effects of lead
acetate on hereditary constitution. Journal of Experimental Zoology. 57(3):409-439.

Sonneborn, T. M. 1931. McDougall's Lamarckian experiment. American Naturalist. 65:541-
550.

Sonneborn, T. M. 1932. Experimental production of chains and its genetic consequences in
the Ciliate Protozoan Colpidium campylum. Biological Bulletin. 63:187-211.

Sonneborn, T. M. and R. S. Lynch 1932. Racial differences in the early physiological ef-
fects of conjugation in Paramecium aurelia. Biological Bulletin. 62:258:293.

Sonneborn, T. M. and R. S. Lynch 1934, Hybridization and segregation of Paramecium aurel~-
ja. Journal of Experimental Zoology. 67(1):1-72.

Sonneborn, T. M. 1937. Sex, sex inheritance and sex determination in Paramecium aurelia.
Proceedings of the National Academy of Science. 23(7):378-385.

Sonneborn, T. M. 1938. Mating types in Paramecium aurelia: diverse conditions for mating
in different stocks; occurrence, number and interrelations of the types. Proceedings
of the American Philosophical Society. 79(3):411-434,

Sonneborn, T. M. 1939. Paramecium aurelia: mating types ant groups; lethal interactions;
determination and inheritance. American Naturalist. 73:390-413,

Sonneborn, T. M. 1942. Inheritance in Ciliate Protozoa. American Naturalist. 76:46-62.

Sonneborn, T. M. and R. V. Dippell 1943. Sexual isolation, mating types, and sexual res-
Beta: to diverse conditions in variety 4, Paramecium aurelia. Biological Bulletin.
85(1): 36-43.

Sonneborn, T. M. 1943. Gene and cytoplasm. I. The determination and inheritance of the
killer character in variety 4 of Paramecium aurelia. Proceedings of the National Aca-
demy of Science. 29: 329-338.

Sonneborn, T. M. 1943. Gene and Cytoplasm. II. The bearing of the determination and in-
heritance of characters in Paramecium aurelia on the problems of cytoplasmic inheri-
tance, pneumococcus transformations, mutations and development. Proceedings of the
National Academy of Science. 29: 338-343.

Sonneborn, T. M. 1945. The dependence of the physiological action of a gene on a primer
and the relation of primer to gene. American Naturalist. 79:318-339.
10

Sonneborn, T. M. and R. V. Dippell 1946. Mating reactions and conjugation between varie-
ties of Paramecium aurelia in relation to conceptions of mating type and variety.
Physiological Zoology. 19:1-18.

Sonneborn, T. M. 1946. Experimental control of the concentration of cytoplasmic genetic
factors in Paramecium. Cold Spring Harbor Symposium on Quantitative Biology. 11:236-
255.

Sonneborn, T. M. 1947. Recent advances in the genetics of Paramecium and Euplotes. Advan-
ces in Genetics. 1:264-358. Academic Press.

Sonneborn, T. M. 1947. Developmental mechanisms in Paramecium. Growth Symposium. 11(4):
291-307.

Sonneborn, T. M. 1948. The determination of hereditary antigenic differences in genically
identical Paramecium cells. Proceedings of the National Academy of Science. 34:413-
418,

Sonneborn, T. M. 1949. Beyond the gene. American Scientist. 37:33-59.

Sonneborn, T. M. and A. LeSuer 1948. Antigenic characters in Paramecium aurelia (variety
4): determination, inheritance and induced mutations. American Naturalist. 82:69-78.

Sonneborn, T. M. 1949. Ciliated Protozoa: cytogenetics, genetics, and evolution. Annual
Review of. Microbiology: 55-80.

Sonneborn, T. M. 1950. Methods in the general biology and genetics of Paramecium aurelia.
Journal of Experimental Zoology. 113(1):87-148.

Sonneborn, T. M. 1950. The cytoplasm in heredity. Heredity 4(1):11-36.
Sonneborn, T. M. 1950. Heredity, environment, and politics. Science. 111(2890) :529-539.
Sonneborn, T. M. 1950. Cellular transformation. Harvey Lecture Series. 44:145-164.

Sonneborn, T. M. 1951. The role of the gene in cytoplasmic ihheritance. Chapter 14, Gene
tics in the 20th Century, ed. by L. C. Dunn, MacMillan: New York.

Sonneborn, T. M. 1951. Some current problems of genetics in the light of investigations
on Chlamydomonas and Paramecium. Cold Spring Harbor Symposium on Quantitative Biol-
ogy. 16: 483-503.

Sonneborn, T. M. 1952. Problems of tumors in relation to studies on Paramecium. Proceed-
ings II National Cancer Conference: 1139-1147.

Sonneborn, T. M. 1954. The relation of autogamy to senescence and rejuvenescence in Para-
mecium aurelia. Journal of Protozoology. 1:38-53.

Sonneborn, T. M. 1954. Patterns of nucleocytoplasmic integration in Paramecium. Caryolo-
gia, Vol. suppl.: 307-325.

Sonnebomn, T. M. 1955. Heredity, development and evolution in Paramecium. Nature. 175:

Sonneborn, T. M. 1957. Breeding systems, reproductive methods, and species problems in
Protozoa. Rep. from The Species Problem, ed. by E. Mayr. American Association for the
11

Advancement of Science, pp. 155-324.

Sonneborn, T. M. 1960. The gene and cell differentiation. Proceedings of the National
Academy of Science. 46:149-165.

Sonneborn, T. M. 1960. Some comments on the general biological situation with regard to
aging. The Biology of Aging, ed. Strehler, American Institute of Biological Sciences,
publication #60, pp. 279-280.

Sonneborn, T. M. 1960. Tests and critique of some theories of aging. The Biology of Ag-
ing, ed. Strehler, American Institute of Biological Sciences, pp. 281-282.

Sonneborn, T. M. and R. V. Dippell 1960. Cellular changes with age in Paramecium. The Bi-
ology of Aging, ed. Strehler, American Institute of Biological Sciences, p. 285.

Sonneborn, T. M. and M. V. Schneller 1960. Age-induced mutations in Paramecium. The Biol-
ogy of Aging, ed. Strehler, American Institute of Biological Sciences, pp. 286-287.

Sonneborn, T. M. 1960. The human early foetal death rate in relation to age of father.
The Biology of Aging, ed. Strehler, American Institute of Biological Sciences, p.
288.

Sonneborn, T. M. 1960. Enormous differences in length of life of closely related ciliates
and their significance. The Biology of Aging, ed. Strehler, American Institute of Bi-
Ological Sciences, p. 289.

Sonneborn, T. M. and M. V. Schneller 1960. Measures of the rate and amount of aging on

the cellular level. The Biology of Aging, ed. Strehler, American Institute of Biolo-
gical Sciences, pp. 290-291.

Sonneborn, T. M. and M. V. Schneller 1960. Physiological basis of aging in Paramecium.

The Biology of Aging, ed. Strehler, American Institute of Biological Sciences, pp.
283-284,

Sonneborn, T. M. 1963. Does preformed cell structure play an essential role in cell here-
dity? The Nature of Biological Diversity, ed. J. M. Alden, pp. 165-221. McGraw-Hill
Publishing Co.

Sonneborn, T. M. 1963. Implications of the new genetics for biology and man. American In-
stitute of Biological Science Bulletin. 13(2):22-26.

Sonneborn, T. M. 1963. Bearing of protozoan studies on current theory of genic and cyto-
plasmic action. Proceedings XVI International Congress of Zoology. 3:197-202.

Sonneborn, 'T. M. 1964. The differentiation of cells. Proceedings of the National Academy
of Science. 51(5):915-929.

Sonneborn, T. M. 1965. Degeneracy of the Genetic code: extent, nature and genetic impli-
cation. Evolving Genes and Proteins, ed. V. Bryson & H. Vogel, Academic Press, New
York, pp. 377-397.

Sonneborn, T. M. 1965. Editor of The Control of Human Heredity and Evolution. Macmillan
Company, New York.

Beisson, J. and T. M. Sonneborn 1965. Cytoplasmic inheritance of the organization of the
cell cortex in Paramecium aurelia. Proceedings of the National Academy of Science.
12

53:275-282.

Sonneborn, T. M. 1965. Nucleotide sequence of a gene: first complete specification. Sci-
ence 148(3676):1410.

Sonneborn, T. M. 1965. Genetics and man's vision. Proceedings of the American Philosophi-
cal Society. 109(4):237-241.

Sonneborn, T. M. 1965. The metagon: RNA and cytoplasmic inheritance. American Naturalist.
46(907) : 279-307.

Sonneborn, T. M. 1966. Genetics of the Protozoa. Genetika. 11:31-41.

Sonneborn, T. M. 1966. Models for somatic cell variation based on studies of Ciliate
Protozoa. Proceedings of the Mendel Centennial Celebration, Czechoslovakian Academy
of Science, Prague, Genetic Variations in Somatic Cells: 87-97.

Sonneborn, T. M. 1967. The evolutionary integration of the genetic material into genetic
systems. Chapter 19, pp. 375-402 in Heritage from Mendel, ed. R. A. Brink, University
of Wisconsin Press, Madison, Wisconsin.

Sonneborn, T. M. 1969. Man's Choices. In Genetic Engineering, BBC Publication, ed. David
Paterson.

Sonneborn, T. M. 1970. Methods in Paramecium research. In Methods of Cell Physiology,
Vol. 4, pp. 241-339, D. Prescott (ed.), Academic Press.

Sonneborn, T. M. 1970. Gene Action in development. Proceedings of the Royal Society (Lon-
don) Series B 176:347-366.

Sonneborn, T. M. 1973. Ethical issues arising from the possible uses of genetic know-
ledge. In Ethical Issues in Human Genetics, ed. by B. Hilton, D. Callahan, M. Harris
es P. Condliffe, and Burton Berkley. Plenum Publishing Corporation, New York, pp. I-

Sonneborn, T. M. 1974. Ciliate morphogenesis and its bearing on general cellular morpho-
genesis. Actualities Protozoologiques, Vol. 1: 337-355, ed. by P. De Puytorac & J.
Grain. Resume des discussions des tables rondes du 4th Congress International de Pro-
tozoologie, Clermont-Ferrand, France, 1973.

Sonneborn, T. M. 1974. Tetrahymena pyriformis. In Handbook of Genetics, Vol. 2, Chapter
19: 433-467. Edited by R. King, STenun Press. ~ "

Sonneborn, T. M. 1974. Paramecium aurelia. In Handbook of Genetics, Vol. 2, Chapter 20:
469-594, Edited by R. King, Plenum Press.

Sonneborn, T. M. 1975. The Paramecium aurelia complex of fourteen sibling species. Trans-
actions of the American Microscopial Soclety. 94:155-178.

Sonneborn, T. M. 1975. Cell differentiation and conmunication: patterns, problems and
probes. Microbiology 1975: 421-425.

Sonneborn, T. M., 1975. Positional information and nearest neighbor interactions in rela-
tion to spatial patterns in Ciliates. Ann. Biol. XIV, Fasc. 11-12: 565-584.