THE MECHANISM OF SEX-DETERMINATION 45 



At the first spermatocyte division all the chromosomes 

 divide (C). The two resulting cells have eight chro- 

 mosomes, including X and F. At the second division 

 {D) the double chromosomes again divide, but X and Y 

 do not divide. They approach and touch each other, 

 and are carried into the spindle, where they separate 

 from each other when the other ordinary chromosomes 



••oV 

 aoZ 



*!-^ 



B 



divide. Consequently there are formed two kinds of 

 spermatozoa — one containing X and the other Y 

 (Fig. 23, E). 



In the body cells and early germ-tract of the female 

 of lygseus (Fig. 24, A), there are twelve ordinary 

 chromosomes and two sex chromosomes, X and X. 

 After reduction there are seven double chromosomes, 

 the two X's having united when the other chromosomes 



