v] SPERMATOGENESIS AND OOGENESIS 61 



oogonia. They divide a number of times by ordinary mitotic 

 division, in which it is seen that they have the ordinary or 

 somatic number of chromosomes. During these divisions 

 there may again be differentiation among the products into 



FIG. 7. Diagrams of spermatogenesis (a) and oogenesis (b), 



after GREGOIRE. 



Three spermatogonial and oogonial divisions are represented, 

 followed by a growth-phase leading to a primary spermatocyte 

 and oocyte. The growth-phase is at first similar in each sex, 

 but is followed by a second stage in the female not repre- 

 sented in the male. The two successive spermatocyte divisions 

 correspond with the two polar divisions of the egg. 



cells which will give rise to gametes and others which per- 

 form some accessory function, such as, for example, the 

 supply of nourishment to the gametes. After a number of di- 

 visions, which in some animals is definitely fixed and in others 

 probably variable, the spermatogonia and oogonia cease to 



