64 P. C. Williams 



stages of normal atresia, except that this occurs peripherally 

 instead of centrally (Figs. 11-12). This is not merely a 

 result of the predominance of solid follicles because it occurs 

 in the same situation in follicles in which antrum formation is 

 beginning. In the untreated, hypophysectomized animal atresia 

 seems to be quite normal ; even when it occurs in almost solid 

 follicles it starts around the oocyte (Fig. 10). The abnormal 

 form of atresia proceeds to completion in the oestrogen-treated 

 animals and eventually outweighs the stock of rudimentary 

 follicles that were present at hypophysectomy, so that the 

 ovary eventually becomes as atrophic as that of the untreated, 

 hypophysectomized rat. This presumably explains a puzzling 

 thing about the original observation: that oestrogen stimu- 

 lated the ovarian follicles, yet long-continued oestrogen treat- 

 ment of intact rats will produce ovarian atrophy. 



I shall not attempt to explain the peculiarity of this form of 

 atresia, but it may be relevant that Bullough (1942) in his 

 counts of the mitoses in normal follicles in the mouse found 

 that there were more in the central layers of the granulosa 

 than in the peripheral layers, and suggested that it was the 

 oestrogen in the follicular fluid that was responsible for this. 

 In the oestrogen-treated, hypophysectomized rat it is fair to 

 assume that oestrogen is concentrated at the external surface 

 of the follicle. The degenerative process has been stated to be 

 initiated in dividing cells of the membrana granulosa (Salazar, 

 quoted by Corner, 1932). 



A further point that may be relevant is that this type of 

 atretic fragmentation has been described as being present in 

 almost all follicles of oestrogen-treated, hypophysectomized 

 rats that have been treated at the same time with chorionic 

 gonadotrophin or androgen (Gaarenstroom and de Jongh, 

 1946). 



These findings demonstrate that the membrana granulosa is 

 a true transient tissue. Its fate may be to degenerate or to be 

 transformed into another transient tissue — the corpus luteum. 

 The life of the membrana granulosa in the rat is limited 

 to about five days. I think I have proved that oestrogen 



