Regenerative Capacity of Ovarian Tissue 41 

 which Aron and his collaborators now report in the guinea 

 pig, and which we can confirm from personal observation 

 One often finds m the ovarian cortex of this animal, cells 

 frequently crowded together in nests, which seem to be either 

 oogonia or oocytes. What appear to be phases of meiosis can 

 be recognized, although it is by no means simple to identify 

 all the stages which Aron et al. have described (cf. Figs. 1-4) 

 On the other hand, they do not describe oogonia undergoing 

 mitosis, their view being that oogonia, having been formed 

 by a straightforward metamorphosis of quiescent germinal 

 elements in the germinal epithelium, become directly trans- 

 formed into oocytes. Aron et al. also believe (a) that this 

 presumed neoformation of oocytes varies considerably from 

 animal to animal; (b) that until the animals are about 2\ 

 years old, when it usually becomes negligible or ceases, the 

 process is independent of age; (c) that the process does not 

 vary cyclically; (d) that its intensity is not affected either by 

 gonadotropin or hypophysectomy; (e) that it does not occur 

 more vigorously in one ovary after its fellow has been removed; 

 and (f ) that it is not affected by the injection of oestrogen. 



What is significant about these observations is not that 

 Aron et al. were able to observe phases of meiosis in nests of 

 oocytes, but the fact that they failed to find any evidence of 

 mitosis of oogonia, and that they were consequently forced to 

 conclude that the latter are formed from the direct transfor- 

 mation of dormant cells in the germinal epithelium. Others 

 before them, as well as ourselves, have also failed to observe 

 mitosis in presumed germinal elements in the guinea pig ovary 

 (e.g. cf. Myers, Young and Dempsey, 1936; Evans and Swezy, 

 1931). It is also worth noting that the cellular formations 

 (e.g. nests of "oocytes ") seen in the guinea pig ovary have lent 

 themselves to a variety of interpretations, and that one group 

 of workers, following Loeb (1905, 1932) regard partheno- 

 genetic development of ovarian oocytes as common in this 

 species (e.g. Myers, Young, and Dempsey, 1936), whereas others 

 (e.g. Stockardand Papanicolaou, 1917; Evans and Swezy, 1931; 

 Bacsich and Wyburn, 1945) regard them as a rarity. It is, of 



