SPERMATOGENESIS INSEMINATION 171 



certain of the other cells in the parietal layer of the seminiferous 

 epithelium. 



In male animals which have a rutting season spermatp-_ 

 genesis occurs only at this time. At other seasons of the year 

 the testes remain in a quiescent condition. 1 The periodic 

 activity of the testis is usually correlated with a great increase 

 in the size of that organ (see pp. 24 and 61). 



Spermatogonium. Oogonium. 



Proliferation 

 period. 



Growth period. 



Maturation 

 period. 



FIG. 45. Scheme of spermatogenesis and oogenesis. (After Boveri.) 



The changes which occur in spermatogenesis may be sum- 

 marised as follows : (1) A spermatogonium divides into two, 

 one product of division passing into the second layer of the 

 seminal epithelium and becoming a spermatocyte. (2) A 



1 In some animals the renewal of activity in the testes is associated with 

 the descent of these organs from their position in the abdominal cavity 

 through the inguinal canal and into a cutaneous fold. This is transformed 

 into the scrotum, which lies behind the penis (except in Marsupials, where it 

 is in front). In many Mammals the descent takes place at an early age and 

 is permanent. In others (most Kodents, Insectivores, and bats) the testes 

 are withdrawn into the abdominal cavity after the breeding season is over. 

 This is effected by the cremaster muscle. 



