Endocrine Mechanisms 747 



total removal of the ovaries of pregnant rats sufficient progestin is liberated 

 from the placentae to carry the animals to normal terms. In man this change- 

 over of control from corpora lutea to placenta is considered to occur between 

 the 79th and the 90th day of pregnancy. 



Progestins are not stored in the body; they disappear very rapidly from 

 the blood. Injection of progesterone, or the normal production of this hor- 

 mone, is followed by the appearance of corresponding amounts of a rela- 

 tively inert alcohol, pregnanediol, in the urine. The preponderance of evi- 

 dence points to the progestational endometrium of the uterus as the prin- 

 cipal organ concerned in this conversion, which is principally in evidence in 

 the progestational phases of the sexual cycle and usually ceases on hyster- 

 ectomy. Furthermore, it is greatly reduced by injections of estradiol, which 

 is known to suppress the progestational condition. That the progestational 

 endometrium is not the sole site of the conversion is seen in the excretion of 

 pregnanediol in males, and occasionally in females after hysterectomy. 



The corpora lutea which are formed from the follicles, either without 

 ovulation, as in the case of atretic follicles, or after ovulation, vary consid- 

 erably in the extent of their development. Their greatest development ac- 

 companies gestation; lesser development is associated with lactation and 

 pseudo-pregnancy and the typical corpora lutea of ovulation are still smaller. 

 The corpora lutea are dependent for their development on a supply of LH 

 from the pituitary. Mammals appear to vary considerably in their need for 

 external stimuli to encourage the necessary production of LH. An adequate 

 stimulus has been shown to be copulation in the case of the rabbit. Suckling 

 and lactation have been shown to stimulate its production in the rat, and 

 psychic stimuli, such as the mere presence of a second individual, have been 

 found effective in such animals as the pigeon and the rabbit. That the last 

 stimulus is visual, at least for the pigeon, is demonstrated by the fact that 

 a mirror image is often sufficient to produce the effect. 



While all investigators are agreed that LH is essential for the initial de- 

 velopment of the corpora lutea, there is much controversy as to the endocrine 

 factors participating in the maintenance of these bodies. There appears to 

 be good evidence from a number of types of experiments that the absence of 

 FSH contributes significantly to their maintenance, and that LH is non- 

 essential. Gonadal hormones assist in their maintenance, probably through 

 a suppression of FSH liberation by the pituitary. Comparison of experiments 

 in which the embryos have been removed, leaving the placenta in situ, with 

 those in which the placentae as well are excised, clearly demonstrates that 

 the latter organs yield hormonal material which can operate to maintain the 

 corpora lutea. These results also appear capable of interpretation in terms of 

 suppression of FSH production by placentally-derived gonadal hormones 

 That still other factors may contribute to the total explanation is seen in the 

 observation that the corpora lutea may be maintained even after complete re- 

 moval of the fetuses and placentae, provided the uteri are kept distended by 

 such inert bodies as pellets of paraffin wax. 



The presence of corpora lutea, at least among the higher vertebrates, ap- 

 pears to be associated with viviparity. They are said to be absent in birds 

 and oviparous reptiles, but present in certain pregnant viviparous snakes, 

 such as Crotolus and Bothwps. 



