REPRODUCTION. 475 



comprising the maternal sinuses permeated by the fetal villi ; this is in con- 

 tact upon the fetal side with the thin unmodified chorion covered within by 

 the amnion, and upon the maternal side with the thin relatively unmodified 

 serotina covered without by the uterine muscle. The pure maternal blood 

 brought by the uterine arteries moves slowly through the sinuses and retiree 

 by the uterine veins; the fetal blood is propelled by the fetal hear! 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 villous invest- 

 ment of connective tissue and epithelium. Thus the anatomical 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 role of the placenta 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 e<r<r 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 

 its decidual wall, the segmented ovum continues to take nutriment from its 

 immediate environing cells. It has been suggested, but without much basis 

 of fact, that the uterine glands, which at this time are greatly dilated, may 

 furnish a nutritive secretion for the use of the embryo; but, a priori, it would 

 seem more reasonable that, just as the ovum within the Graafian follicle 

 obtains its food from its surrounding stroma, so within the highly vascular 

 decidua it absorbs directly from the decidual tissue. Bui thai this source 

 soon proves insufficient for the rapid growth is indicated by the early develop- 

 ment of the chorion with its villi and the embryonic vascular system. In 

 the youngest known human embryo,' believed to be scarcely seven days old, 

 the villi are already well marked. From this time onward throughout 

 tation the chorion takes an important pari in the embryonic nutrition, becom- 

 ing, as we have seen, an integral pari of the placenta. The placenta is par 

 excellence the medium of nutritive communication between mother and child. 



Let us consider briefly the needs of the embryo. The fetal energies must 

 be directed almost wholly to the all-iinportani functions of grow th and prepa- 

 ration for the future independent existence. The organism requires, therefore, 

 1 Peters: Verhandlungen der deutschen Qesellschafl fur Gynakologie, 1897, vii. S. 264. 



