HUMAN REPRODUCTION 245 



with the exterior through the urinogenital sinus, bounded by the labia 

 and containing the clitoris, a small sensory organ homologous with the 

 penis of the male. 



Sex hormones. We have seen something of the nature of the endocrine 

 glands and the coordinating role played by the hormones that they 

 produce. The testes and ovaries (gonads) also produce hormones, in 

 addition to their function of nurturing the germ cells. In general, these 

 hormones affect the development of the male and female secondary sex 

 characters, including degree of skeletal development, amount and dis- 

 tribution of hair, depth of voice, size of accessory sex organs, and sex 

 behavior. They also regulate the sex cycles. The principal sex hormones 

 and their roles are discussed below. 



SEX HORMONES IN THE MALE 



Androgenic hormones. From bull testes there has been isolated an active 

 hormone, testosterone, which has pronounced androgenic (promoting masculinity) 

 effects. Testosterone has not yet been found in the testes of other species but is 

 probably present. A very similar substance, androsterone, has been prepared from 

 the urine of the human male and many other mammals. The chemical composition 

 of both these hormones is known and both have been made in the laboratory. It 

 seems probable that androsterone is formed from testosterone during the latter's 

 excretion from the body. More than 30 other similar substances with androgenic 

 effects have been artificially synthesized. There seems to be no doubt that the 

 androgenic hormones are produced by the interstitial cells of the testis, but it is 

 not yet certain that the seminiferous tubules do not also produce them. 



The role of the androgenic hormones is demonstrated by castration and by 

 injecting the hormones into castrated and normal animals. Castrated cocks, bulls, 

 and men (known respectively as capons, steers, and eunuchs), as well as other 

 castrated animals show characteristic effects of the operation. Young males fail 

 to show the normal changes in body form, stature, external sex organs, and amount 

 and distribution of hair at puberty ; they are fatter and less muscular than normal 

 males, and the body remains more like that of the young female. Castrated adults 

 show a diminution of the peculiarly male characteristics, and their sexual be- 

 havior diminishes. That it does not disappear in men is probably due to the fact 

 that such behavior depends partly upon purely psychological elements. Injection 

 of testosterone into young castrated animals results in normal development of 

 the male characteristics, and in castrated adults testosterone restores full sexual 

 behavior. Such treatment does not, of course, enable castrated animals to produce 

 spermatozoa. In caponized cocks the grafting of a testis within the abdominal 

 cavity has the same results as testosterone injection. This has led to the popular 

 belief that the grafting of testes into ageing men will prolong life and restore 

 potency — an illusory hope. Such grafted testes, even when human, fail to become 

 established and soon degenerate. 



Gonadotrophic hormones. The activity of the testis is under the remote 

 control of the gonadotrophic hormones produced by the anterior lobe of the 



