284 H. M. KINGERY 



made to Swift ('14 and '15) and Firket ('14). To mention a 

 few cases among the mammals, Jenkinson ('13) states that in 

 the rabbit the 'primordial germ cells,' originating in the splanchno- 

 pleure of the yolk-stalk, migrate to the gonad and there develop 

 into the definitive ova; he believes that the definitive germ cells 

 of the mouse have a similar origin. He adds, however :^"This 

 mode of origin of the germ cells does not, of course, preclude the 

 formation of others from the cells of the sex-cords, that is from 

 the germinal epithelium, and it is indeed quite possible that this 

 occurs." Kirkham ('16) says that the primordial germ cells in 

 the mouse give rise to the definitive ova, and Rubaschkin ('12) 

 holds that they persist and form the definitive ova in the guinea 



pig- 

 It is evident, then, that this second view is becoming more 



generally accepted. There is, however, a fatal weakness in the 



evidence heretofore presented in its support. Those who hold 



this conception have, for the most part, been content to trace 



these 'primordial germ cells' in or to the germinal epithelium, 



and then assume that they proceed there to develop into the 



definitive ova. The case has been considered proven when these 



cells had been followed into the embryonic ovary. Very few 



have studied the further history of these cells. Swift (loc. cit.), 



who is very positive that the primordial germ cells form the 



definitive ova," carried his investigations only as far as an 



embryo of fourteen days incubation, stating that d' Hollander ('04) 



and Sonnenbrodt ('08) describe the development of these cells 



into definitive ova. As a matter of fact, d' Hollander did not 



* Swift states ('15, p. 450) that v. Berenberg-Gossler "has confirmed the findings 

 of Swift in all the essential points." This is rather misleading, for the fact is 

 that while von Berenberg-Gossler does confirm the actual facts — that large cells, 

 'primordial germ cells', are present in duck embryos of 24 to 32 somites, he does not 

 accept Swift's interpretation of this. He states:— Alles in allem, bin ich der 

 Ansicht, dass das ganze verhalten dieser zellen in hohem grade 

 davor warnt, siefiir keimbahnzellen zu halten, und dass ihre 

 Genese tiberhaupt ihre Geschlechtszellennatur sehr zweifelhaft macht. . . . 

 Als Hauptergebnis meiner bisherigen Untersuchungen sehe ich die Erkenntnis 

 an, dass man von einer Keimbahn bei Sauropsiden nicht mehr 

 reden kann." (Von Berenberg-Gossler, '14, p. 261-262. The emphasis is 

 Jiis.) ■ 



