450 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
low, more males than females are born. This is probably a case of 
differential prenatal mortality. By that we mean that more females 
die unborn than males, because the latter are hardier and stand pre- 
natal malnutrition better. 
e) Sex is determined at the time of fertilization —Perhaps the best 
evidence that sex is determined at the very beginning of development 
is derived from one-egg twins and quadruplets. In the nine-banded 
armadillo practically every female gives birth to quadruplets, four 
essentially identical young being produced in each litter. All in a 
given set of quadruplets are invariably of the same sex, either four 
males or four females. Newman and Patterson have shown that each 
set of quadruplets comes from a single egg which at a very early stage 
divides into four parts to form four foetuses (Fig. 94). The conclusion 
is that sex was determined before the separation took place. Human 
identical twins, also always of the same sex, furnish further evidence 
in favor of very early sex determination. These and numerous other 
similar facts justify the conclusion that sex is determined at the time 
of fertilization. 
THE CHROMOSOMAL MECHANISM OF SEX DETERMINATION 
The well-established case of Drosophila (pp. 411 ff.) will serve as a 
basis for comparison. Many cases similar to that of Drosophila have 
been worked out. The chief differences have to do with the X and Y 
chromosomes. Sometimes the X chromosome is distinct and inde- 
pendent during synapsis and maturation, but cases are known in 
which the X chromosome is attached to the end of one of the ordinary 
chromosomes, and will, of course, always follow this chromosome in 
reduction division. The Y chromosome varies considerably in dif- 
ferent species. Sometimes the Y chromosome and the X chromosome 
are optically indistinguishable. Sometimes the Y element is repre- 
sented by a group of as many as five small chromosomes which keep 
together in a group and always go either one way or the other in the 
reduction division. Again, the Y element may be very small or vesti- 
gial, or, finally, it may be wanting altogether, so that X is an entirely 
unpaired chromosome that goes to one cell only at the reduction 
division. 
In spite of all of these various modifications of the X-Y type of 
chromosomal sex-determining mechanism, the method of producing 
male-forming and female-forming spermatozoa is the same in each 
case. The female gametes all have one X chromosome, while half of 
