286 H. M. KINGERY 



in the cortical region, from cells derived from the germinal 

 epithelium, and from these the definitive oocytes develop; but 

 it is not improbable, at least, that a small number of the 'pri- 

 mordial germ cells' as well, are differentiated into definitive ova. 

 One of his conclusions may be pertinent here:— 



II faut, done, morphologiquement parlant, considerer les gonocytes 

 primaires (primordial germ cells) des Vertebres comme etant un rappel 

 phylogeniqne des gonocytes definitifs des classes inferieurs, notament 

 des Cyclostomes et des Acraniens. L'epuisement graduel, dans la 

 serie phylogenique des elements de cette lignee a necessite 1' apparition, 

 au cours de I'ontogenese, d'lme seconde lignee de gonocytes, moins 

 precoces. ('14, p. 330-331 ) 



In the mouse, the evidence here presented shows that the 

 definitive ova originate from cells of the germinal epithelium 

 by a process of differentiation, and that this process takes place 

 between birth and sexual maturity. The primitive germ cells 

 also arise from the germinal epithelium, in the embryo. But 

 the relations of the 'primordial germ cells' are not clear. Jenkin- 

 son ('13) and Kirkham ('16) both think that they form the de- 

 finitive ova in the mouse. But Kirkham's paper is merely an 

 abstract, unaccompanied by figures, and therefore can not be 

 considered conclusive. Jenkinson devotes a few pages in his 

 book to this question, but is hardly con\dncing. 



It would seem that, in the mouse, there are three possible 

 courses of development open for the 'primordial germ cells' : — 

 1) they may persist and form the primitive germ cells or oocytes 

 and so eventually degenerate; 2) they may, after reaching the 

 germinal epithelium, develop into the definitive germ cells; 

 or, 3), they may degenerate completely. The evidence pre- 

 sented, I think, shows that neither the first nor thp second 

 is the course followed. The objection mentioned above, that 

 no one has traced these cells through to mature ova, also dis- 

 poses of the second possibility. There is, indeed, the further 

 possibility that the 'primordial germ cells,' after entering the 

 germinal epithelium, lose their size and characteristics and be- 

 come indistinguishable from the mesothelial cells. It would 

 be extremely difficult to prove or disprove this, but the burden 

 of the proof would, it seems to me, rest upon any one who sup- 



