Endocrine Mechanisms 741 



the sexual and reproductive impulse and behavior. These latter hormones 

 are known as the gonadal hormones and include the androgens, estrogens, 

 and progestins. A third group we shall call the lactogens. These hormones, 

 of pituitary origin, are essential to the secretion of milk by the crop glands 

 of pigeons and by the mammae of mammals and probably are influential in 

 determining maternal behavior. 



Gonadotropins. It has been known for some years that pituitary deficiency 

 or hypophysectomy in the mammal results in an atrophy of the gonads and 

 certain accessory genital structures, and that these changes can be reversed 

 by pituitary implants or extract injections. The gonadotropins to which 

 these influences of the pituitary are due always exhibit the common prop- 

 erties of proteins. Factors which destroy protein structure effectively inactiv- 

 ate these hormones. It would therefore appear that the gonadotropins are 

 themselves proteins or consist of a prosthetic group intimately associated with 

 a protein to which the specific activities of the hormones are due. 



The anterior lobe of the pituitary is the principal source of gonadotropins. 

 From this site arise two gonadotropins, the follicle-stim-ulating hormone or 

 FSH, and the luteinizing hormone or LH. Another gonadotropin, called 

 chorionic gonadotropin, is known to arise not from the pituitary but from 

 the placenta and to resemble LH rather closely in function. The bulk of 

 the evidence at hand suggests very strongly that these three gonadotropins 

 are three different compounds. 



In the absence of gonadotropins, such as normally accompanies hypophy- 

 sectomy, the ovary shows an arrest of development in young animals and 

 a reduction in size in adults. In the latter instances existing follicles become 

 atretic. FSM in excess, on the other hand, vv^ill stimulate the simultaneous 

 maturation of an excessive number of follicles. In some mammals, such as 

 the rabbit, ferret, etc., ovulation from mature follicles occurs only after mat- 

 ing or after some other effective stimulus. Such stimulation is known to in- 

 duce the liberation of LH, which causes the ovulation. In other mammals, 

 including the human, no special stimulation of the accessory complex is es- 

 sential to induce ovulation. In all mammals LH is normally responsible for 

 the formation of corpora lutea in the ruptured follicles. 



The gonadotropins are not sex specific. They regulate the activity of the 

 testis as well as of the ovary. Removal of the pituitary in the male results in 

 an arrest of testicular function and reduction in size of the testis. Activity of 

 the testes can be maintained in hypophysectomized individuals by pituitary 

 im.plants or extract injection. Many mammals which breed only at certain 

 sharply delimited periods of the year may have their testes rendered unsea- 

 sonably active by treatment with gonadotropins. This has been demonstrated 

 for such species as the ground squirrel, Citellus, and the alpine marmot, 

 Marmota. After the mating season in such species the testes normally recede 

 into the abdomen and spermatogenesis ceases. Administration of pituitary 

 gonadotropin, or even chorionic gonadotropin, found normally only in preg- 

 nant females, will reverse these processes at any season. 



An enlargement of the ovaries of the horned lizard, Phrynosoma, has been 

 obtained by FSH and LH from hog pituitary and by serum from pregnant 

 mares.-^^ 



Female frogs and toads may be induced to ovulate at any time of the 



