ROLE OF THE OVARY IN GESTATION 99 



female is to quiet the active, irritable condition aroused by the estrogenic 

 factor. Progesterone thus serves to neutrahze or antagonize the effects of 

 the estrogenic hormone. A placid condition of the uterus must be maintained 

 during the period immediately following copulation if the fertilized egg is to 

 be cared for within the uterine structure. Large doses of estrogens injected 

 into mammals shortly after copulation prevent implantation of the embryo 

 in all species thus far studied. (See Selye, '48, p. 822.) 



A third effect of the presence of progesterone is the inhibition of the 

 copulatory responses. Immediately following estrus and ovulation, the female 

 dog will fight off the aggressiveness of the male — an aggressiveness which 

 she invited a day or two previously. This change in behavior is introduced 

 by the development of the corpora lutea and the initiation of the luteal phase 

 of the reproductive cycle. Similar anaphrodisiac changes are sometimes men- 

 tioned in the behavior of the human female during the luteal phase of the 

 cycle. Progesterone injections also inhibit the copulatory responses in the 

 ferret (Marshall and Hammond, '44). All of the above-mentioned activities 

 of progesterone thus inhibit or antagonize the condition aroused by estrogenic 

 stimulation. 



However, aside from these immediate metestrous and post-ovulatory changes 

 in behavior induced by progesterone, one of its most essential activities is 

 concerned with the maintenance of gestation or pregnancy. Ovariectomy or 

 the removal of the ovaries at any time during the gestational period in the 

 rat, mouse, and goat results in death and abortion of the embryo. During 

 the first part of pregnancy in the rabbit, the ovaries must be left intact but 

 may be removed in the closing phase without endangering the gestational 

 process. In the human female, and also in the mare, cat, dog, guinea pig, 

 and monkey, the ovaries may be removed during the latter half of pregnancy 

 without danger to the offspring. However, ovariectomy performed in the 

 early stages of pregnancy in these animals, as well as in all other mammals 

 thits far studied, produces abortion (Pincus, '36; Selye, '48, p. 820). The 

 corpus luteum hormone, therefore, is essential in the early phases of gestation 

 in all mammals, and it appears to be necessary during most of the pregnant 

 period in many other mammals. 



It is highly probable that the placenta takes over the elaboration of proges- 

 terone in those mammals where ovariectomy is possible after the first part 

 of pregnancy has elapsed. In the human female the corpus luteum normally 

 involutes at about the third month of pregnancy, but progesterone may be 

 extracted from the placenta after this period. 



Although certain effects of the estrogenic hormone appear to be neutralized 

 (or antagonized) by progesterone during the early phases of reproduction, 

 other effects of estrogen in relation to progesterone are important for the 

 maintenance of the pregnant condition. In this connection the estrogenic 

 hormone appears to suppress some of the growth-promoting effects of proges- 



