6o VERTEBRATE REPRODUCTIVE CYCLES 



and, in the mammals, the time when the antrum forms 

 inside the mass of folUcle cells. Thus it is possible to 

 divide the growth of the primary oocyte into two 

 phases; the primary phase before either yolk or antrum 

 appears and before a response is possible to the presence 

 of F.S.H., and the secondary phase after the appearance 

 of yolk or antrum and after the development of a sen- 

 sitivity to F.S.H. 



It is not yet known whether any hormone affects or 

 controls the growth of the oogonium and of the primary 

 growth phase of the primary oocyte. 



The second effect produced in the ovary by the 

 anterior pituitary hormones is the stimulation of the 

 production of female hormone. Such a hormone is also 

 known as an oestrogen since its presence in sufficient 

 quantity causes the onset of heat, or oestrus. It is usually 

 thought to be the F.S.H. which induces the secretion of 

 this substance, while it is the I.C.S.H. which causes the 

 growth of the interstitial tissue. If this is true it would 

 appear that hormone secretion in the ovary, unlike that 

 in the testis, is not the function of the interstitial cells. 

 This is an unexpected conclusion and further study of 

 the point is clearly needed, particularly since the inter- 

 stitial cells of the ovary have a similar glandular appear- 

 ance to those in the testis. ^'^^ 



Various attempts have been made to locate the 

 source of the ovarian hormone and, as regards the 

 mammals, one well-known theory is that the site of 

 secretion lies in the theca interna, which is one of the 

 cell layers in the follicle vfslV^' i** However, even if this 

 is true it seems certain that the hormone can also be 

 secreted elsewhere. If all the ova and follicles of the 

 mouse are destroyed by X-rays, the oestrous cycle, 

 which depends on oestrogen production, continues 

 unchecked.!^* Thus the interstitial cells may also secrete 

 female hormone, as in the male they are thought to 

 secrete the male hormone. 



Among the egg-laying vertebrates there is no evidence 



