748 Comparative Animal Physiology 



Relaxin. Relaxin, a non-steroidal substance, insoluble in fat solvents, is 

 found within the corpora lutea and can be isolaied from the blood of preg- 

 nant rabbits. It is capable of relaxins^ markedly the pelvic girdle of a guinea 

 pig in 6 to 12 hours after a single injection. Piogesterone and estradiol induce 

 the secretion of relaxin in female rabbits unless they are castrated and hys- 

 terectomized. The latter observations indicate these organs to be essential to 

 this response to the gonadal hormones. 



Lactogenic Hormones. The production of milk is an adaptation of all 

 mammals and such birds as the pigeon for postpartum nutrition of their 

 young. Hence it is not surprising to find that there is a considerable influ- 

 ence of gonadal hormones on milk-producing glands of the organism. The 

 mammary glands of the mammalian fetus become enlarged under the influ- 

 ence of the hormonal complex of the maternal blood supply, but regress on 

 parturition and typically remain so until sexual maturity, when the animal's 

 own hormonal supply becomes adjusted for their development. The mammae 

 of some mammals, such as the monkey, Macacus, develop fully under the 

 administration of estrogens alone. Other species, such as the rabbit, require 

 administration of both estrogens and progestins for similar development. 

 Since in many species mammary growth cannot be induced by the foregoing 

 hormones after hypophysectomy it would appear that at least one additional 

 substance, a lactogen from the pituitary, is also essential to normal mammary 

 development. 



Whatever the nature of the hormonal complex required for the normal 

 development of the mammary glands, it is clearly evident that lactogen 

 from the pituitary is essential to stimulate the actual secretion of milk. This 

 principle in the mammal has been called galactogen or prolactin, although 

 some investigators prefer to restrict the latter term to the analogous prin- 

 ciple in the pigeon, which stimulates the production of pigeon's-milk by 

 the crop gland. Lactating mammary glands cease activity at once after hypo- 

 physectomy, but may be maintained by administration of lactogen and 

 either adrenotropin or extracts of the adrenal cortex. Normal adrenal func- 

 tioning is thus also essential to continued lactation. 



In the normal regulation of lactation it is generally believed -that the estro- 

 gens present in large amounts during gestation inhibit lactogen liberation. 

 With the disappearance of the estrogens on parturition the lactogen appears 

 in the blood. Strongly supporting this hvpothesis is the well known observa- 

 tion that injection of estrogens will rapidly terminate mammary secretion. 

 The stimulus of suckling, through some neural mechanism, seems also to 

 stimulate the liberation of lactogen, at least in many species. 



Control of Reproductive Cycles and Behavior. The pituitary gland, 

 through the production of gonadotropins, is the principal endocrine organ 

 through which reproductive activities are governed. We, therefore, with 

 reason, look to a study of factors controlling this organ to supply us with 

 fundamental information as to the mechanism of control of reproductive 

 rhythms and adaptive reproductive responses of the organism to its external 

 environment. 



The pituitaries of young mammals are bipotential organs which normally 

 develop into either a male or female type, under the influence of the par- 

 ticular gonad present. The gland is typically a relatively larger organ in the 



