4 
SIDNEY I. KORNHAUSER 
spermatocyte. In dividing again into spermatids, this second 
spermatocyte (fig. 1, a) gives rise to X-bearing cells which form the 
female-determining spermatozoa. The sister second spermato- 
cyte which did not receive an X-chromosome gives rise to the 
two male-determining spermatozoa. A zygote which receives 
two sets of autosomes and two X-chromosomes is a female; a 
zygote which receives two sets of autosomes and one X-chromo- 
some is a male. Since in general both types of sperm are formed 
in equal numbers, the chances of a male- or female-determining 
spermatozoan reaching the egg in the process of fertilization are 
equal, and in general 50 per cent of the resulting zygotes are male 
and 50 per cent female. 
The foregoing case is the simplest mechanism known. While 
this is a fundamental type, still there are many variations of the 
mechanism. Thus the X-chromosome may have a Y partner 
in the male cells. If n = one set of autosomes in a given animal, 
then we have the following combination in this case : 
2 n + XY = male, and 2n + 2X = female 
In the spermatogenesis of such animals, two spermatids receive an 
X-chromosome and two a Y-chromosome, the latter being the 
male-determining spermatozoa. In other cases the X-chromo- 
some may be represented by several discrete components ; and it 
may or may not have a Y-chromosome associated with it in the 
male cells. Thus in Gelastocoris , an hemipteron, the male is rep- 
resented by 2n + 4 X -f Y and the female by 2n -{- 8X. ^^n” 
here equals fifteen, so the male diploid number is 35, and the 
female diploid number is 38. 
Detailed studies of the sex chromosomes have brought out 
many interesting facts. The X-chromosome in some animals is 
attached to a certain autosome, as in Culex or in Ascaris meg ah- 
cephala. When paired, as in the female somatic cells or in 
oocytes, the X-chromosomes behave similarly to autosomes in 
mitosis and in the mitotic divisions and also in syndesis. Con- 
trary to this, the unpaired X-chromosome of the male acts rather 
unlike an autosome in the spermatocytes. It fails to form a 
leptotene thread, generally appearing as a conspicuous karyo- 
