In Pursuit of the Gene

CELL BIOLOGY © 157
mosomes, McClung expanded the search for an accessory chromosome in a
variety of different insect groups. In January 1900 he reported that he had
found it in every case, noting that it came in a great variety of sizes and
shapes and that it was this, he believed, that had been responsible for the
great confusion that had surrounded it. 38 With growing confidence in the
reality of the accessory chromosome, McClung mentioned his suspicion
that it might behave strangely in sperm cell formation. At a division in
which the ordinary chromosomes appeared to go to both poles, the accessory
chromosome went to only one. “I hope by the aid of more favorable
material,” he wrote still withholding final judgment, “to reach a conclusion
concerning the matter.” 39
In April 1900 Sutton published his first study on the development of
the testicular cells in the lubber, providing strong confirmation of the reality
of the accessory chromosome. 40 His paper, though, dealt only with the
early stages of the maturation of the sperm cells and did not shed any new
light on the idea that the accessory chromosome was distributed to only
half of the maturing sperm cells (spermatids). 41
One year later, on January 1, 1901, McClung submitted the third and
most remarkable of his papers on the accessory chromosomes to the Journal
of Morphology. 42 After weighing the evidence, he was now fully persuaded that
the accessory chromosome was passed to only half of the daughter cells in
the final stages of sperm formation. Not only had McClung reassessed the
facts surrounding the accessory chromosome, but as suggested by the title
of his paper, “The Accessory Chromosome: Sex Determinant?” he was now
prepared to speculate on its significance. Assuming the chromosomes were
functionally distinct, he deduced that the two kinds of sperm must have a
different influence on the offspring produced from them. But what was
there that might divide a generation into two even parts? The solution to
this problem, when it occurred to him, was inescapable: “A careful consideration
will suggest that nothing but sexual characters thus divides the
members of a species into two well-defined groups, and we are logically
forced to the conclusion that the peculiar chromosome has some bearing
upon this arrangement.” 43 This was the first example in which a particular
chromosome had been shown to determine a definite combination of characters,
those for female or maleness.