OOGENESIS IN THE ^\TIITE MOUSE 283 



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To return to the two points raised by them concerning a new 

 formation of egg-cells; — I think my series of ovaries is complete 

 enough to satisfy any requirements. As for their second con- 

 sideration, it seems to be rather an unwarranted assumption; the 

 ultimate fate of the cells under discussion is what should decide 

 their status, and not ''the nuclear metamorphoses characteristic of 

 the first stages of the development of the ovary." Perhaps their 

 promised later chapter will throw some light on this question. 



The problem of the origin of the definitive ova is one which 

 has been of interest to many workers, but is not yet definitely 

 settled. I shall take up this point only briefly, referring to the 

 papers by Swift ('14 and '15) and Firket ('14) for a more detailed 

 discussion. 



The two more important \'iews as to the origin of the definitive 

 germ cells are: (1) that they develop by a process of differentia- 

 tion from the mesothelial cells of the germinal epithelium cover- 

 ing the ovary; and (2) that they develop from 'primordial germ 

 cells' or 'Ureier,' which themselves have had their origin else- 

 where (entoderm) and have migrated to the epithelium of the 

 genital ridge, there to become differentiated into the ova. The 

 first is the older view, and, presented by Waldeyer in 1870, is 

 today held probably by a majority of those who have considered 

 the question. The second conception is comparatively recent 

 and probably grew out of Hoffmann's work in 1893. He showed 

 ('93) that the 'primordial germ cells' or 'Ureier' which Waldeyer 

 and his school believed were differentiated from cells of the 

 germinal epithelium, were, in birds, present in the embryo, long 

 before the germinal epithelium or gonad had appeared, and 

 that they were found far from the site of the future reproductive 

 organ. They were found in entoderm, in splanchnic mesenchjone 

 and between the two, and later migrated to the mesothelial 

 covering of the gonad when it started its development. He 

 believed that the definitive ova were the direct descendants of 

 these 'primordial germ cells.' 



'Primordial germ cells' have been described in a number of 

 forms, including fishes, reptiles, birds, and mammals. For a 

 more detailed discussion and bibliography reference is again 



