988 THE PHYSIOLOGY OF REPRODUCTION. 



portion to which the egg becomes attached is the decidiia serotina, 

 and it eventually develops into the placenta, the organ through 

 which the maternal nutriment is supplied to the fetus. The ovum 

 has made considerable progress in its development before reaching 

 the uterus, having formed amnion and chorion, with chorionic villi. 

 Some of the ectodermal cells in the chorion become specialized to 

 form a group of trophoblastic cells which have a digestive action, 

 and it is suggested that the activity of these cells enables the ovum 

 to become attached to the decidual membrane and to hollow out 

 spaces in which the chorionic papilla become inserted.* The further 

 development of the egg into a fetus, the formation of the decidua 

 graviditatis, and the placenta are anatomical features that need 

 not be described here. Detail? of these structures will be found 

 in works on anatomy, embryology, or obstetrics. On the phys- 

 iological side it has been found that removal of the ovaries, or even 

 destruction of the corpora lutea, shortly after pregnancy has begun 

 brings the process to an end, while a similar operation later in 

 pregnancy has no effect upon the developing fetus or the subsequent 

 act of parturition. It seems, therefore, that the process of implan- 

 tation of the ovum in the uterine mucous membrane and the devel- 

 opment of a placenta are dependent in some way upon the ovaries. 

 The apparent explanation of the connection is given in the hypothe- 

 sis that the corpora lutea, during their rapid development at the 

 beginning of pregnancy, give off an internal secretion which controls 

 or influences in some essential way the processes connected with 

 the fixing of the fertilized ovum.f 



The Nutrition of the Embryo — Physiology of the Placenta. 

 At the time of fertilization the ovum contains a small amount 

 of nutriment in its cytoplasm. The amount, however, in the mam- 

 malian ovum is small and suffices probably only for the initial stages 

 of growth. When the ovum becomes implanted in the decidual 

 membrane of the uterus the new material for growth must be ab- 

 sorbed directly from the maternal blood of the uterus. Within 

 a short time, however, the chorionic villi begin to burrow into the 

 uterine membrane at the point of attachment, the decidua serotina, 

 and the placenta gradually forms as a definite organ for the control 

 of fetal nutrition,, The details of histological structure of this 

 organ must be obtained from anatomical sources. For the purposes 

 of understanding its general functions it is sufficient to recall that 

 the placenta consists essentially of vascular chorionic papillae from 

 the fetus bathed in large blood-spaces in the decidual membrane of 

 the mother. The fetal and the maternal blood do not come into 

 actual contact ; they are separated from each other by the walls of 

 the fetal blood-vessels and the epithelial layers of the chorionic 



* See Minot, "Transactions of the American Gynecological Society," 1904.. 

 t Marshall and Jolly and Fraenkel, loc. cit. 



