PLACENTAL HORMONES 263 



come implanted in the walls of the uterus. If after the egg is implanted in the 

 walls of the uterus the pituitary is removed, the fetus is resorbed. During the 

 later period of pregnancy, however, the pituitary may be removed with no 

 deleterious effects. Thus it would seem that the placenta can take over to a 

 great extent the hormone control during late pregnancy. As a matter of fact, 

 the secretions of the pituitary — LH and FSH — stop at about the second 

 month of human pregnancy, and do not begin again until after birth. During 

 this time the placenta secretes a gonadotropic hormone which maintains the 

 corpus luteum. Thus the embryo and placenta become almost completely in- 

 dependent of the maternal hormones and form a self-differentiating unit. 



In summary (see Fig. 169), the pituitary secretes FSH, which stimulates 

 follicle growth (1). Follicle growth results in the production of estrone (2), 

 which brings about preovulatory changes in the uterine wall. Ovulation then 

 occurs. Then LH from the pituitary stimulates growth of corpus luteum (3) 

 and this results in the formation of progesterone and some estrone. These 

 hormones produce the postovulatory changes in the uterus (4). If the egg is 

 fertilized (5) it becomes embedded in the uterine wall as a result of the 

 supply of progesterone and estrone (6) from the corpus luteum. The de- 

 veloping egg in turn secretes a gonadotropin (7), which maintains the 

 corpus luteum. In later pregnancy the progesterone and estrone from the 

 corpus luteum are necessary for the attachment of the placenta (8). There 

 may possibly be a relaxin (9), which relaxes the pelvic ligaments. In late 

 pregnancy neither the corpus luteum nor the pituitary is necessary and the 

 placenta itself secretes progesterone, an estrogen, and a gonadotropic hor- 

 mone. 



This picture of hormone control is not a complete one. It is very difficult 

 to get a complete picture. An attempt has been made here to combine infor- 

 mation obtained from experiments upon various types of animals, but the 

 observations from different animals are not always consistent. The outlines 

 of the true picture seem clear enough, however, and we have tried to stay 

 within these general outlines. The specific relations of the various hormones 

 in different mammals vary tremendously. For example, progesterone inhibits 

 the contractions of the uterine wall and estrone stimulates these contractions. 

 It might be expected then, that by injection of large amounts of estrone con- 

 tractions would be initiated in the uterine wall, and these contractions would 

 produce abortion. This situation is true in some animals; in mice injections of 

 estrone lead to abortion. However, it is not true in humans and in some other 

 forms. Thus there is a great deal of variation in the response to these hor- 



