260 EMBRYOLOGY 



of the corpus luteum (Fig. 169, 3) . If this hormone called LH (or ICSH), is 

 injected, the follicles become filled with lutein cells but do not grow. The 

 two hormones secreted by the pituitary, then, act on follicle growth and on 

 growth of the corpus luteum. These hormones are proteins, and LH has been 

 purified (see Chap. 2). 



Once the control of the pituitary over the ovarian cycle is understood, the 

 next question is how this cycle causes the change from preovulatory condition 

 to postovulatory condition to menstruation in the uterus. The ovary secretes 

 several hormones. At this point we shall consider two of them. One extract of 

 the ovary yields a substance which, when injected into ovariotomized animals, 

 produces estrus (Fig. 169, 2). If the ovary is taken out of an animal the 

 estrus cycle stops. If the ovarian hormone, estrone, is then injected into the 

 animal the uterine wall thickens, the vaginal epithelium is shed as in normal 

 estrus, and the female becomes receptive to the male. Estrone is produced by 

 the follicle of the ovary. 



A second ovarian extract, made from the corpus luteum and injected after 

 estrone has acted, produces the postovulatory condition (Fig. 169, 4). This 

 is characterized by a thickening of the wall of the uterus due to increase in the 

 secretory activity of the glands. The hormone is called progesterone. The 

 corpus luteum produces both progesterone and estrone. Thus we have the 

 general picture of two hormones from the pituitary controlling the ovarian 

 cycle and two hormones from the ovarian cycle controlling the uterine cycle. 



Pregnancy 



This mechanism runs smoothly and rhythmically until fertilization occurs 

 (Fig. 169, 5). After the human egg is fertilized in the oviduct, it reaches the 

 uterine wall during the postovulatory period and at this time the wall is very 

 thick. The developing egg becomes, first of all, embedded in the uterine wall 

 and later forms a placenta in collaboration with uterine tissues. The embryo 

 is attached to this placenta and the uterine wall, as shown in Figure 169. The 

 first thing which happens after fertilization is that something from the early 

 developing embryo causes the corpus luteum to increase in size and to persist 

 during pregnancy. The substance responsible is a gonadotropic hormone 

 secreted by the chorion of the developing egg (Fig. 169, 7). The corpus 

 luteum becomes enlarged instead of degenerating into scar tissue as it does 

 when pregnancy does not occur. On the right side of Figure 169 the corpus 

 luteum resulting from pregnancy is shown. Thus the embryo secretes sub- 



