595 VERTEBRATE LIFE AND ORGANIZATION 



hormones to develop the secondary sex characters. Hypophysectomy of 

 an aduU results in involution and atrophy of the gonads. It is now clear 

 that there are two gonadotropins, called foiiicle-stimulating hormone 

 (FSH) and luteinizing hormone (LH), and that both are necessary for 

 achieving sexual maturity and for the regulation of the estrous cycle. 

 The effect of follicle-stimulating hormone is primarily on the develop- 

 ment of graafian follicles in the ovaries: it does not produce any signifi- 

 cant release of estrogen. Luteinizing hormone controls the release of ripe 

 eggs from the follicle, the formation of corpora lutea, and the produc- 

 tion and release of estrogens and progesterone. Prolactin, or lactogenic 

 hormone, maintains the secretion of estrogens and progesterone, and 

 stimulates the secretion of milk by the breast. It is effective, however, 

 only after the breast has been stimulated by the proper amounts of 

 estrogen and progesterone. Prolactin induces behavior patterns leading 

 to the care of the young (the "maternal instinct") in mammals and in 

 other vertebrates as well. Roosters treated with prolactin will take care 

 of chicks, taking them to food and water, sheltering them under their 

 wings, and protecting them from predators. The cyclic release of FSH 

 and LH is involved in the control of the estrous cycles of lower mam- 

 mals and the menstrual cycles of primates. The simultaneous adminis- 

 tration of FSH and LH produces much greater effects on ovarian growth 

 than either one alone; similar instances of hormonal synergism have 

 been observed with certain other pairs of hormones. 



The development and functioning of the testis is also controlled by 

 FSH and LH. FSH increases the size of the seminiferous tubules and 

 both FSH and LH are needed for normal spermatogenesis. LH, but not 

 FSH, stimulates the interstitial cells of the testis to produce male sex 

 hormone. 



Extracts of the pituitary liave been prepared which have other 

 effects when injected, and it has been postulated that the gland secretes 

 other hormones in addition to these six. Despite repeated attempts, it has 

 not been possible to separate and purify the agents of these other activ- 

 ities and many investigators now regard them as side effects of one of 

 the known hormones. The insulin antagonist effect of the pituitary, the 

 "diabetogenic hormone," is now believed to be a property of the growth 

 hormone. 



The control of pituitary function, which ensures that the proper 

 amount of each of these hormones will be released at the proper mo- 

 ment in response to the demands of the organism, is indeed complex. 

 Recent research has revealed that the release of each tropic hormone 

 is controlled in part by the level of the target hormone in the circulat- 

 ing blood. The release of ACTH is inhibited by hydrocortisone, the 

 release of thyrotropin is inhibited by thyroxin, estrogens decrease the 

 output of FSH and progesterone decreases the secretion of LH. This 

 provides for a cut-off mechanism so that in a normal animal the secre- 

 tions of the pituitary and its target organs are kept in balance. 



The release of ACTH is also stimulated by epinephrine. This is 

 apparently a direct effect, for it is observed when the pituitary is re- 

 moved from its normal site and transplanted to the eye. Epinephrine 



