OOGENESIS IN THE WHITE MOUSE 285 



study material from chicks more than twenty days after hatching 

 (v. p. 161), and his evidence on this point is therefore not con- 

 clusive. Sonnenbrodt studied material from chicks just hatched 

 to hens several years old; he states that the egg-cells in the ovary 

 at hatching are oocytes, and that they develop into the defini- 

 tive ova. He apparently accepts d'Hollander's views as to the 

 origin of these oocytes. Although neither of these works alone 

 is conclusive on this point, together they serve to support Swift's 

 view. 



Nevertheless, it has not been conclusively demonstrated that 

 these 'primordial germ cells' develop into the definitive ova. 

 In fact, there is weighty evidence to show that they play no 

 part in the formation of the latter. Firket ('14) states that 

 Dustin, Kuchekewitsch, and Allen admit that these 'primordial 

 germ cells' degenerate totally or partly in Amphibians and Sau- 

 ropsids; Allen and Popoff admit a degeneration in the testis and 

 Skrobansky in the ovary of mammals. Kirkham ('16) states 

 that in the male mouse the 'primordial germ cells' all degenerate, 

 while in the female they form the definitive ova. Von Wini- 

 warter and Sainmont ('08) hold that these 'primordial germ 

 cells' in the cat are temporarily hypertrophied cells and have 

 nothing to do with the process of oogenesis. 



Firket ('14) studied material from chick embryos from eighty- 

 two hours incubation to hatching, and in addition, the ovaries from 

 chicks just hatched up to young hens of six months, an age when 

 the hens of most breeds are sexually mature and have begun to 

 lay. He believes that his work proves the sexual or genital 

 nature of these 'primordial germ cells,' or 'primary gonocytes,' 

 as he prefers to call them. His studies show that the gonocytes 

 of the medullary zone (formed by the first of the two embryonic 

 proliferations frorn the germinal epithelium, as von Winiwarter 

 described for the cat) develop into oocytes which pass through 

 the first stages of the growth period and then degenerate. They 

 have all disappeared in the chick fourteen days after hatching. 

 The oocytes of the cortical zone (second embryonic proliferation) 

 practically all degenerate, although he states that he can not 

 be sure that they aU do. There is a new formation of germ cells 



