64 VERTEBRATE REPRODUCTIVE CYCLES 



and it is evidently this that stimulates the mitotic 

 activity of the follicle wall. The theory can therefore be 

 advanced that while the pituitary induces the active 

 secretion of follicular fluid and of oestrogen, it is the 

 oestrogen that causes the real growth of the follicle. 



This conception of the action of the oestrogenic 

 hormone is still a new one, but another of its actions is 

 now well established. When a Graafian follicle bursts to 

 release the egg within it, the follicular fluid is also 

 spilled from the ovary. As it oozes out, the oestrogenic 

 hormone which it contains comes into contact with the 

 germinal epithelial cells on the ovary surface, and 

 there is a burst of mitosis. In the germinal epithelium 

 of one mouse ovary as many as 2,000 cell divisions may 

 occur within a period of 9 J hours, ^ and this results in the 

 production of a new crop of oogonia, which pass into 

 the ovary to start their cycle of growth.^. 52, 57 Thus 

 whenever eggs are shed from the ovary a new cycle of 

 tgg production starts. 



In the egg-laying vertebrates there is, of course, no 

 follicular fluid, but the egg-laying period, when oestrogen 

 production is probably at a maximum, is also a time of 

 high mitotic activity in the germinal epithelium. Among 

 fishes this has been seen in the minnow,^ '^ among 

 amphibians in the common frog,^® among birds in the 

 starling,^* ^nd among marsupials in the opossum.^e 



One final point about the oestrogen s is that, like 

 testosterone, they have a depressing effect on the rate of 

 pituitary secretion. However, it is said that unless 

 excessive quantities are present, the immediate eflPect 

 of oestradiol is to depress the production of F.S.H. only, 

 while that of I.C.S.H. increases. 



The second substance produced by the normal 

 ovary is the male hormone testosterone, which has 

 already been described. At the moment its function in 

 the female is unknown. Sometimes, in pathological 

 cases, excessive quantities are produced, and in this way 

 a hen may develop such secondary sexual structures as a 



