262 EMBRYOLOGY 



the corpus luteum and later an estrogenic substance is also secreted by the 

 placenta. Thus ovulation is inhibited until after birth. 



The corpus luteum during early pregnancy has a number of functions. 

 First of all, the progesterone from the corpus luteum sensitizes the uterine 

 wall for implantation of the developing egg (Fig. 169, 6). If progesterone 

 is injected into some animals, the uterine wall becomes very sensitive, so that 

 if it is merely touched or scratched, growths will occur. A second property of 

 progesterone is the inhibition of the contractions of the uterus. The uterine 

 wall remains relatively quiet during most of pregnancy, and it may be the 

 function of progesterone to inhibit muscular contractions. In this respect 

 progesterone is antagonistic to estrone, since the latter stimulates uterine con- 

 traction. The corpus luteum is also necessary for the early development of 

 the embryo after it has become implanted (Fig. 169, 8). Removal of the 

 corpus luteum during early human pregnancy results in death and abortion 

 of the embryo. This effect is a direct consequence of the removal of pro- 

 gesterone and estrone, which build up the uterine wall and keep it built up 

 during early pregnancy. 



In addition to progesterone and estrone, the corpus luteum of sheep 

 secretes a third hormone, which acts on the pelvic ligaments (Fig. 169, 9). 

 These ligaments relax and allow for a larger pelvic cavity for the developing 

 embryo. If this hormone, called relaxin, is injected into males it also produces 

 a relaxation of the pelvic ligaments. 



Although the corpus luteum is necessary for both the implantation of the 

 blastocyst and the early development of the human embryo, it is not necessary 

 during the later period of pregnancy. The corpus luteum — indeed, the whole 

 ovary — can be removed after the first three to four months of pregnancy, and 

 the embryo will go to term. There must, therefore, be some other source of 

 the ovarian hormones, progesterone and estrone. And this second source is 

 found in the placenta itself. 



Placental hormones 



The placenta secretes progesterone. It also secretes some estronelike sub- 

 stance, an estrogen. Since the placenta contains progesterone and an estrogen, 

 during later pregnancy the placenta itself takes over the function of the 

 corpus luteum and the ovary. In a similar way the placenta appears to take 

 over the function of the pituitary itself. If the pituitary gland is removed in 

 very early human pregnancy, no implantation occurs. The egg does not be- 



