528 VERTEBRATE LIFE AND ORGANIZATION 



The removal of the testis (castration) of an immature male pre- 

 vents the development of the secondary sex characters. A castrated man, 

 a eunuch, has a high-pitched voice, beardless face, and small genitals 

 and accessory glands. Castration was practiced in the past to provide 

 guardians for the harem and sopranos for choirs. Many kinds of do- 

 mestic animals are castrated to make them more placid. The injection 

 of testosterone into a castrated animal restores all of the sex characters 

 to normal. The anal fin of the male mosquito fish, Gambusia, is dif- 

 ferentiated into a penis-like organ used to transfer sperm to the 

 female. This fails to develop if the fish is castrated but appears if the 

 castrate male or the female is treated with testosterone. 



It should be emphasized that males produce female sex hormones 

 (estrogens) and that females produce androgens in considerable amounts. 

 The normal diflferentiation of the sex characters is a function of a balance 

 between the two. 



The failure of the testes to descend normally from the body cavity 

 to the scrotal sac, called cryptorchidism, produces sterility but has little 

 or no effect on the production of testosterone. Microscopic examination 

 of an undescended testis shows that the cells in the seminiferous tubules 

 regress, but the interstitial cells are normal. The cells of the seminiferous 

 tubules are particularly susceptible to heat, and the temperature of the 

 body cavity, 3 or 4 degrees higher than that of the scrotal sac, destroys 

 them. It is probable that the elevated temperature during a prolonged 

 fever makes a man sterile for some time. In many wild animals the 

 testes remain in the body cavity except during the breeding season, when 

 they descend into the scrotal sac. 



The removal of the pituitary causes regression of both the inter- 

 stitial cells and the seminiferous tubules of the testis. Androgen secretion 

 is decreased and the secondary sex characters regress. Normal develop- 

 ment and spermatogenesis of the cells of the seminiferous tubules ap- 

 parently requires the combined action of FSH, LH and testosterone. 

 The administration of excessive amounts of testosterone or estrogen may 

 produce regression of the testes, presumably by inhibiting the release 

 of FSH and LH from the pituitary. 



The cyclic growth and regression of the testis in animals with 

 periodic breeding seasons appears to be mediated via the pituitary. Such 

 animals have very low amounts of gonadotropin in the nonbreeding 

 season. Changes in the temperature or in the amount of daily illumina- 

 tion produce stimuli which are mediated by the brain and hypothalamus 

 to induce gonadotropin secretion by the pituitary and consequent 

 growth and functional state of the testis and secondary sex characters. 



255. The Ovaries 



The ovaries of vertebrates are endocrine organs as well as the 

 source of eggs; they produce the steroid hormones estradiol and pro- 

 gesterone. Some mammalian ovaries produce a third hormone, the pro- 

 tein relaxin. 



