ClikOi\IOSOMES, HEEEDITV ANJ) SIEX. 
497 
are of siicli a natare that it would liardly be exaggeratic>u to 
claim them as proving that hypothesis. 
Very early in the present century cases were observed in 
insects in which one chromosome differed conspicuousl}^ from 
the rest in spermatogenesis, especially in taking no part in 
the spireme stage, but behaving as a, ‘^chromatin-nucleolus.’’ 
It was then found that these species had an odd number 
of chromosomes in the male, an even number in the female, 
and that the difference was due to the presence of a pair 
of such “accessory” chromosotnes in the female, and only 
one in the male. The “accessory” iii the male has no 
mate, and goes over undivided in one (usually the first) 
spermatocyte division, but divides equationally in the other, 
with the result that of the four spermatids derived from each 
spermatogonium, two possess it and two are without it. In 
the female the two accessories pair and separate, so that one 
is present in every egg-nucleus. If all the spermatozoa are 
equally functional, half the zygotes will have one accessory, 
the other half two, and the natural inference was drawn that 
the former become males, the latter females. Such cases 
were at first known only in Orthoptera and Hemiptera ; they 
have now been observed in most orders of insects except 
Lepidoptera and Hymenoptera, and in many other groups, a 
list of which is given later. Not all species of these groups, 
however, show this difference between the sexes, and doubt 
was cast on the sex-determining function of the accessory 
chromosome on the ground that its existence should be 
expected in every species which has two sexes. The next 
step was made by Wilson (65). He found that in certnin 
Hemiptera the spermatogonia contained two “accessories,” 
one of which was noticeably larger than the other, and that 
in consequence half the spermatids contained a large one, 
half a small, but that the females of these species had two 
large ones, so that each mature egg contains one. To avoid 
confusion he named these unequally paired bodies “ idio- 
chromosornes,” and the unpaired body of the species previously 
described, a “heterotropic” chromosome. He further dis- 
