398 EVOLUTION, GENETICS, AND EUGENICS 



divides into four parts to form four fetuses (Fig. 77). 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 CHEOMOSOMAL MECHANISM OF SEX DETERMINATION 



In two previous chapters (chaps, xxiv and xxix) descriptions of the 

 typical modes of chromosomal sex determination have been given. In 

 order to facihtate a clear understanding of this important matter, 

 it seems well to recapitulate one typical instance. Perhaps the best- 

 known instance of sex determination is that of Drosophila melanogaster, 

 already described and figured (Fig. 78) by Babcock and Clausen. In 

 this insect the female body cells and the unmaturated germ cells are 

 characterized by the presence of two sex chromosomes (X-chromo- 

 somes), which are shown in black at the top of the left-hand column 

 of the accompanying figure. The chromosomes are readily dis- 

 tinguishable by being of medium size and straight. The male body 

 cells and immaturated germ cells (top of right colimin) are just like 

 those of the female except that there is substituted for one of the 

 X-chromosomes a hook-like chromosome, known as a Y-chromosome. 

 Now in the process of maturation of the germ cells, which results in 

 the formation of gametes with the haploid or half-somatic number of 

 chromosomes, each of the eggs (female gametes) receives an X-chromo- 

 some. AU eggs are therefore alike in their chromosome content, in- 

 cluding the sex chromosome. The case is different on the male side; 

 for two kinds of gametes are formed, one kind with an X-chromosome 

 and the other with a Y-chromosome. These are formed in exactly 

 equal numbers, as one of each is produced at every reduction division. 

 Each egg must be fertilized by one or the other of these two kinds of 

 sperms, and in the long run as many eggs will receive an X-chromosome 

 as will receive a Y-chromosome. Those that receive an X-chromosome 

 wiU be characterized by having two X-chromosomes, which is the 

 typical female condition, and thus a new female individual is started 

 in life; while those that receive no X-chromosome, but a Y-chromo- 

 some, wiU have the XY composition characteristic of the male sex, 

 and will give rise to males. The female sex may thus be designated 

 as XX and the male sex as XY. We have shown for Drosophila the 

 exact mechanism that operates in determining whether an individual 



