OOGEKESIS IN THE WHITE MOUSE 289 



come from this source. In both these last cases however, it may 

 be questioned whether such a formation of egg-cells before birth 

 is strictly comparable to a proliferation of cells after birth and 

 before sexual maturity. 



Felix ('12) in describing the development of the ovary in man 

 says that there is an early proliferation of cells from the germinal 

 epithelium, forming the 'epithehal nucleus.' Later, a 'young 

 cortical zone' becomes differentiated, whether from the outer 

 part of the epithelial nucleus, or by a renewed activity on the 

 part of the epithelium resulting in a proliferation of cells, he is 

 unable to say with certainty, although inclining toward the 

 former view. With the exception of this possible proliferation 

 from the germinal epithelium, there is no addition of epithelial 

 cells to the ovary. After the tunica albuginea is formed (in 

 embryos of 180 mm. length) no cells can be added to the interior 

 of the ovary. Apparently there is no possibility of a new forma- 

 tion of ova from the germinal epithelium. 



From the foregoing, it is clear that a new formation of germ 

 cells after birth is not of very general occurrence. It may be 

 that, as von Winiv/arter and Sainmont ('08 a) suggest, such a 

 'formation of ova has been overlooked because a careful study 

 has not been made of a complete series of ovaries. Certainly, 

 the study of a complete series of ovaries between birth and sexual 

 maturity and into adult life, has shown that in the mouse there 

 is such a proliferation of germ, cells after birth. 



It is possible that the criticism may be made that the egg-cells 

 described above, in the germinal epithelium of postnatal ovaries, 

 as developing primary oocytes, are, in reality, cells which have 

 already passed through the earlier stages of development in the 

 embryonic ovary, and are, perhaps in von Winiwarter's diplotene 

 or dictye stages. I have gone over this carefully and am con- 

 fident that such is not the case. In the first place, these egg- 

 cells differ in many particulars from the cells described as diplo- 

 tene or dictye, and are not to be confused with them. Secondly, 

 these post natal oocytes are actually smaller than the primitive 

 oocytes of the later stages (cf. figs. 21 and 22 with 28 to 31). 

 Thirdly, intermediate forms can be observed between these cells 



