REPROD UCTION. 



913 



Amnion. 



Chorion. 



inasmuch as the villi have important respiratory functions. The bulk of the 

 placenta is this intravillous portion, of spongy consistence, comprising the 

 maternal sinuses permeated by the 

 fetal villi ; this is in contact upon 

 the fetal side with the thin un- 

 modified chorion covered within 

 by the amnion, and upon the 

 maternal side with the thin rela- 

 tively unmodified serotina covered 

 without by the uterine muscle. 

 The pure maternal blood brought 

 by the uterine arteries moves 

 slowly through the sinuses and 

 retires by the uterine veins; the 

 fetal blood is propelled by the fe- 

 tal heart along the umbilical cord 

 within the allantoic arteries and 

 through the villous capillaries, and 

 returns by the allantoic vein. The 

 two kinds of blood never mix, but 

 are always separated by the thin 

 capillary walls and their thin vil- 

 lous investment of connective tissue 

 and epithelium. Thus the anatom- 

 ical conditions for ready diffusion 

 are present, and this is the chief 

 means of transfer of nutriment and 

 oxygen from mother to child, and 

 of wastes from child to mother. 

 The physiological r6le of the pla- 

 centa is, therefore, an all-important 

 and complicated one. The placenta is, technically, the nutritive organ of 

 the embryo. 



Nutrition of the Embryo. We have seen that a fundamental and most 

 striking difference between the minute human ovum and the large egg of the 

 fowl lies in the relative quantity of food contained in the two. The fowl has 

 retained the primitive habit of discharging the ovum from the maternal body, 

 and discharges within its shell at the same time sufficient food for the needs of 

 the developing chick. Evolution has endowed the human mother, in common 

 with other mammals, with the peculiar custom of retaining the offspring within 

 her body until its embryonic life is completed, and of doling out its nutriment 

 molecularly throughout the period of gestation. The store of nutritive deuto- 

 plasm with which the egg leaves the ovary is, therefore, only sufficient for the 

 early segmentative activities. Within the Fallopian tube absorption from the 

 surrounding walls doubtless goes on. Arrived in the uterus and imbedded in 



58 



FIG. 315. Diagram of the placenta (Schafer) : s, pla- 

 cental sinuses, into which project the fetal villi, con- 

 taining the red fetal vessels ; d. s, decidua serotina ; s. p, 

 spongy layer, and m, muscular layer, of the uterus ; a, 

 uterine artery, and v, uterine vein, opening into the 

 placental sinuses. 



