8 THE EGGS OF MAMMALS 



(1909) describe a degeneration of the primordial germ 

 cells in the cat ovary and their replacement by ingrowths 

 from the germinal epithelium from three and one-half to 

 four months after birth {cf. Kingsbury, 1913 and 1914a; 

 Foulis, 1876 and Balfour, 1878). De Winiwarter (1910) 

 observed the same phenomena in human ovaries. In the 

 rat embryos Firket (1920) observed a secondary proliferation 

 following degeneration of the first generation of germ cells. 

 Kingery (1917) in a detailed study of oogenesis in the mouse 

 found that the definitive oocyte arose from secondary pro- 

 liferation begun at three to four days before birth and last- 

 ing until thirty-five to forty days post partum. He found 

 no evidence for oogenesis after puberty. In the rabbit 

 Buhler (1894) also found only prepubertal ovogenesis. 



Simkins (1923 and 1928) questions the vahdity of the 

 term primordial germ cells, going so far as to state that in 

 the human embryo they are not large wandering cells at 

 all but large liquefied areas surrounding degenerating nuclei. 

 He attributes complete autonomy to the genital ridge. 

 Kohno (1925) recognizes primordial germ cells in the hu- 

 man embryo but declares their origin is in lateral plates of 

 the mesoderm whence they reach the gonad via the gut 

 epithelium and mesentery. Hargitt (1925) also denies the 

 peritoneal origin of the germ cells in rat embryos declaring 

 that large differentially staining cells are found throughout 

 the embryo in the epithelium, mesoderm, ectoderm, gut 

 entoderm and extra embryonic tissues. The disappearance 

 of these cells he attributes to division, not to migration 

 into the genital ridge. 



A number of more recent investigators have observed a 

 more or less continuous proliferation of ova from the ger- 

 minal epithelium throughout life. The chief modern protag- 

 onists of this view are Robinson (1918), Arai (1920a and 6), 

 Allen (1923), Papanicolou (1925), Butcher (1927), Swezy 

 (1929a, 1933a and h) and Evans and Swezy (1931). Their 

 histological studies are essentially confirmations of earlier 

 observations on post natal ovaries (Pfluger, 1863 — cat; 



