608 VETERINARY PHYSIOLOGY 



it is the interstitial cells which are the active part ; for, after 

 complete degeneration of all the cells of the Graafian follicles, 

 the graft of ovarian tissue still produces the development of the 

 sexual characters. 



The transplantation of ovaries into young castrated male rats 

 and guinea-pigs has led to the development of female characters, 

 such as excessive growth of the nipples, to the secretion of milk, 

 and in rats to characteristic sexual reflexes. 



The evidence is thus quite clear that the gonads exercise 

 a direct influence upon the somal cells through an internal 

 secretion. 



But certain peculiar modifications in the development 

 of the sexual characters have been recorded which appear 

 inexplicable on any theory of the action of an internal secretion. 

 Bond has described a pheasant with male plumage on 

 one side and female plumage on the other, and a similar 

 condition has been recorded in a bullfinch. In the pheasant 

 an atrophic ovo-testis was present. It is, of course, inconceiv- 

 able that an internal secretion could have had this bilateral 

 action. In insects there is also evidence that the sexual 

 characters develop after castration of the caterpillar. 



In the feinale the ovaries not only act in the young in 

 determinino- the development of the sexual characters, but in 

 adult life they have an important influence on — 



(1) The Course of Pregnancy. — In the bitch if the ovaries 

 are removed early in pregnancy the ovum does not become 

 embedded in the mucous membrane of the uterus. The cells of 

 the corpus luteum exercise an influence on the uterus which 

 brings this about. This structure is formed when the Graafian 

 follicle ruptures and discharges its ovum. It is produced essen- 

 tially by an enormously increased growth of the cells of the 

 zona granulosa, which become loaded with lipoids. If the ovum 

 is fertilised the corpus luteum grows to a large size ; if the 

 ovum is not fertilised it grows to a less extent. When the 

 corpus luteum is formed, any irritation of the uterine mucosa 

 may cause the development of the typical tissue for the 

 embedding of the ovum. 



