THE OVARY IN MAMMARY-GLAND DEVELOPMENT 107 



During pregnancy, the actual secretion of milk is inhibited by the estrogenic 

 hormone produced by the ovary and the placenta. The role of estrogen as 

 an inhibitor of lactation is suggested by the fact that, after lactation has started 

 following normal parturition, it is possible in the cow and human to suppress 

 milk flow by the administration of estrogens. After parturition, however, 

 estrogen is no longer present in sufficient amounts to suppress the secretion 

 of milk, and the mammary gland begins to function. (In the fur seal a post- 

 partum estrus with ovulation follows a short time after parturition. However, 

 the amount of estrogen produced by this reproductive cycle is not sufficient 

 to curb lactation.) The neurohumoral reflex, or "suckling reflex," produced 

 by the sucking young appears to maintain the flow of milk over a period of 

 time. Probably this reflex causes a continuous discharge of the lactogenic hor- 

 mone from the anterior lobe of the hypophysis. 



Another theory of mammary-gland development maintains that estrogen 

 stimulates the anterior pituitary gland to release mammogen, which causes 

 development of the duct system, and estrogen plus progesterone induce a 

 second mammogen which stimulates lobule-alveolar development. The lac- 

 togenic hormone produces the actual secretion of milk. The ovary thus as- 

 sumes considerable importance in controlling the (morphological) develop- 

 ment of the mammary glands in mammals, particularly in those forms in 

 which the functional condition of the ovary is maintained throughout most 



Fig. 59. Stages in the reproductive cycle of the human female and its pituitary-ovarian- 

 endometrial relationships (Cf. fig. 53). (Compiled from various sources in the literature.) 

 (a) As shown at the extreme right of the figure, a fall in the level of estrogen and proges- 

 terone in the blood stream, either or both, is associated with endometrial necrosis, bleed- 

 ing, and discharge (menstruation), (b) The lowering of the estrogen level is associated 

 with a new outflow of the follicle-stimulating hormone (FSH), as shown at the right of 

 the figure, (c) In the left side of the figure, the influence of FSH induces egg follicles, 

 probably several, to grow. Antral spaces appear and enlarge. The presence of a small 

 amount of the luteinizing hormone (LH) together with FSH stimulates the secretion of 

 estrogen by the ovarian tissues, possibly by the follicles and interstitial tissue between 

 the follicles, (d) In consequence, the estrogen level rises in the blood stream, and 

 menstruation subsides by the fourth day. (e) The continued influence of estrogen pro- 

 duces endometrial growth, and probably increases the outflow of LH from the pituitary 

 (fig. 53). It is probable, also, that the increased estrogen level stimulates a release of 

 the luteotrophic hormone from the pituitary, which in turn stimulates the formation of 

 a small quantity of progesterone by either the interstitial tissue of the ovary or in old 

 corpora lutea. (f) Some of the developing egg follicles degenerate, while one continues 

 to develop, (g) The elevation of estrogen suppresses the outflow of FSH as indicated 

 by the heavy broken line to the left, (h) The elevated level of estrogen together possibly 

 with small amounts of progesterone evokes an increased outflow of LH and LTH as 

 indicated by the heavy broken line to the right, (i) LH and FSH bring about ovulation 

 at about the fourteenth day. (j) LH causes development of corpus luteum. (k) LTH 

 elicits secretion of progesterone by corpus luteum. Possibly some estrogen is secreted 

 also by corpus luteum. (1) Progesterone and estrogen stimulate added development of 

 endometrium, (m) In the absence of fertilization of the egg, the corpus luteum regresses, 

 with a subsequent fall of progesterone and estrogen levels in the blood stream, terminating 

 the cycle and permitting a new menstrual procedure. 



