746 Comparative Animal Physiology 



Estrogens are also produced by the testes of males. In fact the tissue with 

 the highest known estrogen yield is the stallion testis, in which estrone is 

 the substance present. The testicular source appears to be, as for the andro- 

 gens, the interstitial glandular cells. Estrogens have also been extracted from 

 the adrenals. 



Estrogens are very rapidly inactivated in the living body. Studies on the 

 relation between site of transplantation of ovarian tissue and the physio- 

 logical actions of the implants indicate the liver to be the principal site of 

 destruction. Estrogen-containing blood passing through the liver reappears 

 largely free of active concentrations of the hormone. Destruction of hepatic 

 cells by carbon tetrachloride permits the hormone titer in the blood to rise 

 with expected effects on estrus of the animal. Liver slices inactivate estradiol 

 in vitro through the action of some CN-sensitive mechanism. Progestin, on 

 the other hand, diminishes the rate of estrogen inactivation and thus permits 

 a higher titer to be maintained in the blood, from which much is then able 

 to escape into the urine. 



The kidneys and liver are the principal organs of estrogen excretion, al- 

 though the relative importance of the two organs is not yet known. 



The organ and tissue changes induced by estrogens are characterized by 

 large variations in degree of responsiveness of the different cells and tissues 

 to the hormone, thereby giving rise to gradients of response. Also character- 

 izing the responses is a very high degree of reversibility of the changes when 

 the concentration of the hormone declines. Such reversibility of the changes 

 is most evident in many of the secondary and accessory sex character changes 

 paralleling the reproductive cycles in many species. 



As with the androgens, the specific threshold and character of the re- 

 sponses are determined by the inherent cellular characters, as can be seen in 

 the unchanged character of the response as the individual tissues are trans- 

 planted to novel sites within the body. The relation of the response to the 

 genetic constitution of the species is seen in reciprocal transplantations of 

 feather-bearing skin between the sexes in fowl, resulting in the characteristic 

 feather types for the host sex. 



PROGESTINS. The progestins include all those compounds exhibiting an 

 action on the organism like that produced by progesterone. These substances 

 are concerned primarily with those changes in the organism associated with 

 pregnancy and parturition. The chief sources of progestin are the i corpora 

 lutea of the ovary. The source would therefore appear to be cells derived 

 from the granulosal layer of the follicle, although some investigators believe 

 that thecal tissue participates in lutea formation. It is possible that other cells 

 of the ovary also contribute progestins, with perhaps the same cells which 

 at other times produce estrogens gradually becoming transformed in the re- 

 productive cycle to liberate progestins. 



Progestins are also produced in the adrenals. Not only has progesterone 

 been demonstrated in this organ, but desoxycorticosterone also acts as a pro- 

 gestin. Extracts of adrenals can produce typical progestational alterations in 

 the uterus in rabbits if, as a preliminary, they are treated with estrogen. 



The placenta is an important natural source of progesterone. In the mam- 

 mal the placenta is believed normally to take over after a time the major 

 share of production of the normal progesterone of pregnancy. Even after the 



