McClung, Clarence Erwin

(b. Clayton, California, 5 April 1870; d. Philadelphia, Pennsylvania, 17 January 1946)

cytology, zoology.

Through his studies on the accessory chromosome in insects, Clarence E. McClung contributed significantly to the establishment of the chromosome theory of inheritance. In 1901 and 1902 he pointed out that the accessory chromosome (or X chromosome as it is sometimes called) was possibly the nuclear element responsible for determining sex. Thus he was among the first to offer evidence that a given chromosome carried a definable set of hereditary traits.

Of Scotch-Irish descent (i.e., Scots who moved to Ireland), McClung’s ancestors had come to the United States in 1740 and settled in Lancaster County, Pennsylvania. His father, Charles Livingston McClung, was a civil and mining engineer, and his mother, Annie H. Mackey, the daughter of a physician. Because his father’s business required the family to move about (mostly through the West and Midwest), Young McClung’s schooling was sporadic. By the time he was ready to enter high school, however, the family had settled in Columbus, Kansas (in the mid-1880’s) and McClung’s intellectual abilities began to show. He became especially interested in science; from his father he learned surveying, and from working in an uncle’s drug store, he learned pharmacy. Following this latter interest, he entered the University of Kansas School of Pharmacy in Lawrence (1890), and completed the pharmacy course in two years (receiving his Ph. G. in 1892). For a year he taught chemistry and pharmacy at the university, and in the fall of 1893 enrolled in the College of Liberal Arts.

As an undergraduate his interests shifted from chemistry to zoology, particularly through the influence of S. W. Williston, who encouraged McClung’s natural mechanical bent and allowed him to work in the histology laboratory learning special techniques. Williston also entrusted McClung with teaching part of his course in histology during one semester. McClung received his B.A. in 1896, and immediately entered graduate school at Kansas, receiving his M.A. in 1898, and Ph.D. in 1902. During his graduate years he spent one semester at Columbia University with the cytologist Edmund Beecher Wilson, and one summer (1898) working with William Morton Wheeler at the University of Chicago. Both of these men were interested in the nature and behavior of chromosomes, and it was through their influence that McClung’s attention was directed to these nuclear elements.

While still a graduate student at Kansas he was appointed assistant professor of zoology, and later of histology and animal morphology (1898–1900); in 1901 he was made associate professor of zoology and head of the zoology department. He served as curator of the university’s paleontological collections (1902–1912), and was acting dean of the Medical School (1902–1906). In 1906 he became professor of zoology, a post he held until 1912 when he accepted a call to become head of the zoological laboratories at the University of Pennsylvania, where he remained until his retirement in 1940. During the academic year 1940–1941 he served as acting chairman of the department of zoology at the University of Illinois, and in 1943–1944 was acting chairman of the biology department at Swarthmore College (Pennsylvania). Always interested in teaching, McClung introduced a variety of pedagogical innovations during his career. At Kansas, where he taught introductory biology, he deemphasized memorization and encouraged students to think through problems on their own. And at Pennsylvania, he served on many university committees concerned with curriculum reform and matters of basic educational policy.

McClung’s biological work covers several distinct areas of interest: paleontology (on which he wrote some early papers in 1895, 1905, and 1908), technical microscopy and microscopy techniques, and studies on chromosomes. The latter area includes his most important contributions.

By the 1890’s it had become clear to most biologists that the chromosomes were somehow involved in the processes of heredity. Both Wilhelm Roux and August Weismann had suggested theoretical roles for the chromosomes as hereditary determiners, while a host of microscopists had carefully detailed the movements of chromosomes in the mitotic and meiotic divisions. The constancy of chromosome number for any species was recognized; and the number of chromosomes for the cells of higher animals and plants was found to be even, suggesting that equal numbers come from the egg and from the sperm. It was also known that the chromosomes divide longitudinally at each somatic division, and that in the formation of gametes (sperm or egg cells) the chromosome number is reduced by one-half (each sperm or egg receiving only half the total number characteristic of the species). Yet there were also a number of facts now part of our common understanding of heredity which were unknown at that time. It was not known until after 1901 or 1902 that the chromosomes exist in definite pairs (homologs). Furthermore, it was thought that when the chromosomes reappear at the end of interphase (just before the cell begins its division cycle), they are linked together end-to-end as one continuous thread (called a spireme). As the division cycle begins, the thread was thought to break up into the number of chromosomes characteristic of the species. The most basic question still unanswered was, in terms of the hereditary information they carry, does each chromosome differ qualitatively from the others, or are they all basically the same, each bearing a full complement of hereditary information? If it were possible to identify a given hereditary trait (or set of traits) with any specific chromosome, this basic question would be answered. The chromosomes would then appear to have individuality, i.e., each would appear to differ from the others, and would control a different trait or traits.

In 1891 Hermann Henking had noted the peculiar meiotic behavior of an unusual chromosome in spermatocytes of the small fire wasp (Hemipteran) Pyrrhocoris. This chromosome, which Henking called a “chromatin body” and labeled “X,” did not seem to pair up with a partner chromosome at prophase I of meiosis, thus seeming to lead a peculiarly separate existence. Furthermore, Henking noted that the male of the species had an uneven number of chromosomes, while the female had an even number. In spermatogenesis one-half of the sperm received this “X” element (thus ending up with twelve chromosomes), while the other half did not receive it (thus ending up with only eleven chromosomes). Similar “accessory chromosomes” were reported for other species of insects, including the long-horned grasshopper, Xiphidium fasciatum, as observed by McClung in 1899. It was not until McClung’s suggestive papers of 1901 and 1902, however, that the significance of these unpaired “X” elements was fully realized. Most important, McClung’s work had a crucial, though indirect significance for the question of the individuality of the chromosomes.

When McClung was in Chicago in 1898, Wheeler suggested that he study spermatogenesis in the grasshopper Xiphidium; Wheeler himself had been working on oogenesis in this species, and wanted someone to do the complementary studies on males. In 1899 McClung published his first set of observations on the “X” element which he observed clearly in grasshopper spermatocytes; because it was unpaired he coined the term “accessory chromosome” to replace Henking’s rather vague designation of “X.” In two longer papers (1901 and 1902) McClung went beyond the cytological observations of his predecessors. He saw in the behavior of the accessory chromosome a clear mechanism for understanding how sex could be determined. McClung started with two notions (1) that the chromosomes were the bearers of hereditary information and (2) that sex was to some extent at least determined by heredity. Thus it was logical to conclude that sex must in some way be determined by the chromosomes. The accessory chromosome, McClung pointed out, fits all the requirements for being a sex determiner: it is present in all cells of the organism until they are fully formed, thus serving to control the development of sexual characteristics in the adult; and it is present in one-half the gametes of the sex that bears it (e.g., males). Thus, at fertilization, one-half the zygotes will contain the accessory, and one-half will lack it. Now, McClung reasoned, sex is the only hereditary characteristic which divides a species into two equal groups. It is quite possible, then, that the accessory chromosome is the hereditary element which determines this difference.

Although his theory proposed that sex was determined at the moment of fertilization, McClung did not hold that this was necessarily true for all species, or that the environment did not sometimes influence the development of the hereditary predisposition. In humans, he felt sex was determined at conception, as evidenced by the fact that identical twins are always of the same sex. But in other species this might not be the case. He took a broad (and from today’s perspective too indulgent a view of the environmental theory) approach to this question when he wrote in 1902:

Sex, then, is determined sometimes by the fact of fertilization and can not be subsequently altered. But between this extreme and the other of marked instability [i.e., totally dependent on the environment] there may be found all degrees of response to environment. It must accordingly be granted that there is no hard-and-fast rule about the determination of sex, but that specific conditions have to be taken into account in each case.

We know today that environmental factors do have some influence on sex ratios in some species; but they do not have the fundamental role which McClung and some of his contemporaries often assigned to them.

It should be noted that McClung made two additional errors in his theory of sex determination. Because he gave such prominence to the influence of environmental conditions, he believed in a theory popular at the time called “selective fertilization” —the idea that the egg could “choose” whether it was fertilized by a sperm bearing an accessory, or by one lacking the accessory. We know today that the egg is incapable of carrying out any such selectivity. Furthermore, through a miscount of the number of chromosomes in the female of the insects he was studying, McClung thought that a sperm bearing an accessory chromosome (the X) would be male-determining. In fact it is, as we know now, female-determining (a sperm bearing either no accessory or a Y-chromosome is male-determining).

MeClung posed his ideas about the role of the accessory only as a working hypothesis, tentative and perhaps suggestive, but not as a proof of the chromosomal nature of sex determination. The evidence was largely circumstantial, resting upon the observed parallelism between the presence or absence of the accessory and the sexual differentiation into males or females. His hypothesis proved to be very stimulating, however. Within a few years (1905) E. B. Wilson and Nettie M. Stevens, independently, showed that for most groups of animals males carried one accessory chromosome (the genotype being designated as X, XO, or XY, “Y” indicating a second element, the Y-chromosome, in many species found associated with the X), and females two accessory chromosomes (the genotype being designated XX). In some species they found that the relationship was just reversed: males had two accessories (XX) and females one (XO or XY). Their work provided a sound and systematic basis for the chromosomal theory of sex determination.

McClung’s work, and subsequently that of Wilson and Stevens, substantiated the concept of the individuality of the chromosomes. By associating the inheritance of one set of traits (sexual) with one particular chromosomal element, the MCClung theory suggested that each chromosome was different from the others, governing one specific set of characteristics. At just about the same time, the concept of chromosomal individuality received considerable support from other quarters: particularly from the work of Boveri with polyspermy in the sea urchin (1902), and that of T. H. Montgomery (1901) and W. S. Sutton (1902, 1903).

Through the concept of chromosomal individuality, McClung’s work led directly to the correlation of the chromosome theory with Mendel’s newly publicized laws of heredity (rediscovered in 1900), The parallelism between the separation of maternal and paternal members of each chromosome pair in meiosis and Mendel’s postulate of segregation of heritable factors was picked up by both Montgomery (1901) and Sutton (1902, 1903). Sutton had been a graduate student of McClung’s at Kansas in 1901, and gained both interest and insight into the chromosome question from his teacher. At the time he wrote his own important papers on Mendelism, however, Sutton had moved on to Columbia University as a student of E. B. Wilson. McClung’s studies on the accessory chromosome came at a fortuitous time in the history of genetics. They pulled together disparate information by suggesting a purpose for the otherwise inexplicable movements of the chromosomes in general and of the accessory in particular during spermatogenesis. They gave a new and fruitful means for understanding the inheritance of sex by providing a mechanism for explaining the 1:1 male-female ratio observed for most species. And lastly, they set the stage for relating Mendel’s abstract “factors” to real material bodies (chromosomes) in the cell nucleus.

Like many biologists at the time, McCIung saw heredity and development as inextricably linked. Heredity operates, he maintained, by controlling the cell’s metabolic functioning. And since that functioning depends upon environmental circumstances (for availability of raw materials, for example), it was clear that an individual’s ontogeny was molded by hereditary potentialities interacting with specific environmental conditions. This was a consciously epigenetic view, conditioned in part by reaction to the prevalence of particulate theories of heredity in the past (such as those of Haeckel or Weismann) which placed all emphasis on heredity. To McCIung, the most important arena in which further understanding of the relationships between heredity and development could be worked out was cytology. Biochemical knowledge was too scanty at the time to make his approach a feasible one for understanding how cells grew and differentiated during embryonic development. But cytology was accessible, and should be used to explore these fundamental questions:

The apprehension of large principles of organization should therefore be our aim, and I have no doubt that once an understanding of the cytological changes in the body of an animal during its ontogeny is reached, we shall have solved, as far as it is possible for us to do, some of the larger problems of heredity and development that have become our scientific inheritance [1908].

In addition to his studies on chromosomes, McClung also made contributions to techniques of biological staining and to the design of microscopes. Particularly noted was the “McClung model” microscope, which had an improved mechanical design. He was also the author of a series of papers on photomicrography and a Handbook of Microscopical Technique (1929), and he pioneered in the use of simple, new designs in laboratory equipment for introductory biology courses.

Besides his teaching and research, McClung served in a number of administrative posts outside the university. In 1913 he was appointed a trustee of the Marine Biological Laboratories, Woods Hole, Massachusetts (he had taught part of the embryology course there in the summer of 1903), and in 1914 a member of the investigative staff. In 1917 he became chairman of the Zoology Committee of the Division of Biology and Medicine of the National Research Council (NRC), and in 1919 the first chairman of its newly created Division of Biology and Agriculture (NRC). In the latter post he initiated plans for a comprehensive biological abstracting service, which eventually led to the publication of Biological Abstracts (published by the Union of American Biological Societies, of which McClung became president of the board of trustees, 1925–1933). As chairman of the Division of Biology and Agriculture, he also initiated plans for standardizing biological stains, eventually giving rise to the Biological Stain Commission. He served as the managing editor of the Journal of Morphology from 1920 until 1946, and the associate editor of Cytologia from 1930 onward.

McClung held memberships in all of the important zoological societies. These include American Morphological Society (1901), American Association for the Advancement of Science (1902; fellow, 1908); American Society of Zoologists (secretary, Central Branch, 1905; president, Central Branch, 1910; president, national organization, 1914); American Philosophical Society (1913); American Society of Naturalists (1913; president, 1927); Philadelphia Academy of Sciences (1914); Wistar Institute of Anatomy and Biology, advisory board (1914); American Association of Anatomists (1914); and the Washington Academy of Sciences (1920).

In addition, he received many honors, including membership in several honorary societies. The latter include Sigma Xi (national president, 1917–1921), Beta Beta Beta (president, 1936), and the National Academy of Sciences (1920). He was also the United States representative at the International Biological Congress in Montevideo, Uruguay, in 1930, a goodwill scientific ambassador (sponsored by the Rockefeller Foundation) to Japan (1933–1934), a recipient of the Distinguished Service Citation from the University of Kansas (1941), and a D.Sc. from Franklin and Marshall College (1942).

McClung was known as a congenial and friendly person by both his colleagues and students. He had a broad range of interests involving athletics, English literature, music, photography, and dramatics. Noted as a sensitive and meticulous worker, McClung was described by one student as “an artist in everything he does.” He was married to Anna Adelia Drake of Lawrence, Kansas, on 31 August 1899; they had two children, Ruth Cromwell and Della Elizabeth.

BIBLIOGRAPHY

I. Original Works. The most complete bibliography of McCIung’s writings is at the conclusion to the biographical sketch by Wenrich mentioned below; however, this covers only up until 1940. His most important papers include: “The Spermatocyte Divisions of the Acrididae,” in Kansas University Quarterly,9 (1900),73–100; “Notes on the Accessory Chromosome,” in Anatomischer Anzeiger,20 (1901), 220–226; “The Accessory Chromosomes: Sex Determinant?” in Biological Bulletin,3 (1902), 43–84; “Cytology and Taxonomy …,” in Kansas University Science Bulletin,4 (1908), 199–215; “A Comparative Study of Chromosomes in Orthopteran Spermatogenesis,” in Journal of Morphology,25 (1914), 651–749; and “The Cell Theory— What of the Future?” in American Naturalist,74 (1939),47–53.

II. Secondary Literature The only substantial biographical treatment of McClung was prepared by D. H. Wenrich on the occasion of McCIung’s seventieth birthday: “Clarence Erwin McClung,” in Journal of Morphology,66 (1940), 635–688. Wearich also prepared several shorter treatments after McCIung’s death: American Naturalist,80 (1946), 294–296; and Yearbook, American Philosophical Society (1946), 322–325. These are all, however, truncated versions of the more complete biographical sketch. Further information can be found in a series of short sketches devoted to McClung in Bios,11 (1940), 141–155, by various colleagues.

Garland E. Allen