THE ORIGIN OF THE DEFINITIVE OVA 21 



germinal epithelium. Furthermore, in the hgated ovaries 

 the follicles become necrotic and new ova are proliferated 

 from the germinal epithelium which is relatively unimpaired. 

 Athias (1920) has described proliferation of new ova from 

 the germinal epithelium of transplanted guinea pig ovaries. 

 No attempt has been made to make a quantitative study of 

 the relation between the number of new ova formed and 

 the amount of functional germinal epithelium in trans- 

 planted or fragmented ovarian tissue, but it seems evident 

 that the formation of new ova in such tissue occurs in the 

 germinal epitheUum. Thus in Tamura's material the few 

 cases of degenerated transplants were marked by a complete 

 absence of germinal epithelium. 



It is possible, however, to preserve an intact germinal 

 epithelium with total disappearance of follicles in x-rayed 

 ovaries (Parkes, 1926, 1927a, b and c; Brambell, Parkes and 

 Fielding, 1927a and h). Parkes and his coworkers have 

 described in some detail the replacement of degenerated 

 folUcular tissue by cellular proliferations from the germinal 

 epithelium in the irradiated ovaries of mice. These pro- 

 liferations never give rise to ova, however, though the ovaries 

 seem to retain their hormone-producing capacities as evi- 

 denced by the continuance of oestrus cycles of normal length 

 in the irradiated animals. In the ferret (Parkes, Rowlands 

 and Brambell, 1932) x-ray sterilization is also marked by an 

 obliteration of the follicles and oestrin secretion, whereas 

 in guinea pig ovaries (Genther, 1931, 1934) a transformation 

 to luteal tissue usually occurs with only occasional follicle 

 formation. Brambell (1930) inclines to the belief that the 

 destruction of primordial ova is responsible for the lack of 

 ovogenesis, but it is equally likely that the x-rays affect 

 differentially the ovogenetic and hormone-producing capac- 

 ities of ovarian tissue. It is notable therefore that the pro- 

 liferation of new tissue from the germinal epithelium in 

 x-rayed mice resembles the production of anovular follicles. 

 Hill and Parkes (1931) have attempted to induce germ cell 

 formation in mice with irradiated ovaries by means of in- 



