852 THE PHYSIOLOGY OF REPRODUCTION. 



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. Details of these structures will be found 

 in works on anatomy, embryology, or obstetrics. 



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 villi, 

 but an active diffusion relation is set up between them. Nutritive 

 material, proteid, fat, and carbohydrate, and oxygen pass from 

 the maternal to the fetal blood, and the waste products of fetal 

 metabolism carbon dioxid, nitrogenous wastes, etc., pass from the 

 fetal to the maternal blood. The nutrition of the fetal tissues is 

 maintained, in fact, in much the same way as though it were an 

 actual part of the maternal organism. That material passes from 

 the maternal to the fetal blood is a necessary inference from the 

 growth of the fetus. The fact has also been demonstrated repeat- 

 edly by direct experiment. Madder added to the food of the mother 

 colors the bones of the embryo. Salts of various kinds, sugar, drugs, 

 etc., injected into the maternal circulation may afterward be de- 

 tected in the fetal blood. But we are far from having data that 

 would justify us in supposing that the exchange between the 

 two bloods is effected by the known physical processes of os- 

 mosis, diffusion, and filtration. The difficulties in understanding 

 the exchange in this case are the same as in the absorption of nour- 

 ishment by the tissues generally. It is perhaps generally assumed 

 that the chorionic villi play an active part in the- process, func- 

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



