THE MECHANISM OF SEX-DETERMINATION 47 



In the body cells of the male there are sixteen chro- 

 mosomes (Fig. 25, A). After reduction there are nine 

 chromosomes — seven in a ring and two in the middle 

 (B). The seven are the fused pairs or double chro- 

 mosomes ; the two in the middle are the sex chromo- 

 somes that have not fused. 



OncapettZc^ 9 



:m 



10 





D 



Fig. 26. 



The evidence for this interpretation is circumstan- 

 tial but sufficient. 



At the first reduction division all nine chromosomes 

 divide (C). Just before the second division the two 

 central chromosomes come together and remain in 

 contact {DD'). All the double chromosomes then 

 divide, while the two sex chromosomes simply sepa- 

 rate from each other, so that there are eight chromo- 

 somes at each pole (DE). 



