Endocrine Mechanisms 745 



active substances and androgens of somewhat lower activity, such as andro- 

 sterone. Estrogens are also commonly produced in the process. These prod- 

 ucts of metabolic conversion are excreted by the kidney. 



The relation of androgens to the development of the characteristic male 

 suspensor organs, gonopodia, and to the smaller body size in male poeciliid 

 fishes has been carefully analyzed.^^^ Castration of males during the meta- 

 morphosis of the anal fin into a gonopodium results in immediate cessation 

 of the process. Application of androgenic hormones to castrated males or to 

 normal females can induce full development of the male characters which 

 are typical of the species. It has been shown that progressively larger con- 

 centrations of hormones are required for consecutive steps in the develop- 

 ment of the gonopodium, with a concentration of one part of hormone in 

 4.2 X 10^** parts of water being sufficient to induce the first step. The gono- 

 podium normally develops under the influence of a gradually increasing con- 

 centration in the blood of androgen from the fish's own testes. A certain op- 

 timum concentration is required for normal development of each step, higher 

 concentrations inhibiting growth and inducing precocious differentiation. 



ESTROGENS. Estrogen is a term applied to any substance which will pro- 

 duce the characteristics of normal estrus, including cornification in the vagina, 

 in an adult mouse. The estrogens include a number of naturally occurring 

 compounds which differ slightly from one another chemically and in ef- 

 fectiveness: estradiol, estrone, estriol, equilenin, and equilin. The princi- 

 pal functions of the estrogens are to stimulate the growth and functioning 

 of those accessory reproductive organs which are characteristically female, 

 to contribute to the production of the conformation of the body which ren- 

 ders it typically female, including the characteristically smaller size for such 

 species as man, and to influence in a characteristic manner the psychology 

 and behavior of the organism. 



The exact source of estrogen within the ovary is still the subject of con- 

 siderable controversy. Earlv observations that the follicular fluid was rich in 

 estrogen led to the hypothesis that the granulosa cells were the sites of 

 estrogen formation. This view has been supported by demonstration that 

 these cells contain estrogen. However, destruction of the follicles by x-ray 

 does not deprive the ovary of its estrogen-producing capacity under the in- 

 fluence of gonadotropins. After intensive treatment with x-radiation, the 

 ovary comes to consist of little other than interstitial tissue. Gonads of fetal 

 horses before follicle formation have proved rich in estrogens. There appears 

 therefore to be convincing evidence that interstitial tissue, including the 

 thecal cells, produces the hormone, and that there is also a possibility that 

 the granulosa and luteal cells manufacture it. It is often difficult to establish 

 that a substance is actually formed within a tissue in which it is found, in- 

 stead of arriving there secondarily. 



The placenta, both the maternal and the embryonic portion, contains 

 abundant estrogen of much the same character as ovarian estrogen. Further- 

 more, oophorectomy in certain pregnant mammals does not result in signifi- 

 cant change in the term of pregnancy or in any permanent reduction in the 

 amount of estrogens excreted in the urine. In such cases the symphyses also 

 become separated normally, a change ordinarily conditioned by joint action 

 of estrogen and progestin. 



