450 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



low, more males than females are born. This is probably a case of 

 differeiitial 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. 410 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 



