38 



HEREDITY AND SEX 



for the female (Fig. 20). In their earUer history the 

 germ-cells of the male divide with the full number of 

 chromosomes characteristic of the male, which may be 

 one less chromosome than in the female. The early 



Fig. 20. — A-B, somatic cell division with four chromosomes. C-H, 

 the two matviration divisions to produce the four cells {H) that become 

 spermatozoa. (After Wilson.) 



germ-cells then cease to divide for a time, and begin 

 to grow, laying up yolk and other materials. At this 

 time the chromosomes unite in pairs, so that the num- 

 ber appears to be reduced to half. Later two divisions 

 occur (Fig. 20, D-H), in one of which the united chro- 

 mosomes separate. The male germ-cells differ, how- 



