CIRCULATION IN THE EMBRYO. 



661 



the decidua serotina, and their walls become atrophied, being finally 

 represented only by epithelial cells covering the capillary blood- 

 vessels which have come from the allantois. The blood-vessels 

 in the decidua serotina become 

 converted into blood-spaces, 

 sinuses, to which the uterine 

 arteries carry blood, and from 

 which the uterine veins carry 

 the blood away. It will be seen, 

 therefore, that the fetal blood- 

 vessels are surrounded by the 

 maternal blood in the uterine 

 sinuses, the two fluids being 

 separated only by the thin wall 

 of the fetal capillaries, through 

 which the interchanges of oxygen 

 and carbon dioxid take place, 

 and also the passage of the nutri- 

 tious material to supply the 

 growing fetus, and in the reverse 

 direction pass the effete products 

 to be eliminated. The structure 

 which performs all these im- 

 portant offices is the placenta, 

 made up of both maternal and 

 fetal tissues. It seems hardly 

 necessary to say that the blood 

 of the mother and that of the 

 child never come in contact, 

 but are always separated by the walls of the fetal capillaries. 



At birth the placenta is cast off, arid by the contraction of the 

 uterine muscular tissue the mouths of the maternal blood-vessels 

 are closed, and thus hemorrhage is prevented. The blood which 

 escapes during a normal labor is that which was in the sinuses. 

 The functions of the placenta are thus seen to be threefold nutri- 

 tive, respiratory, and excretory. 



Circulation in the Embryo. Vitelline Circulation. During 

 the earliest part of human fetal life the contents of the ovum 

 supply the growing embryo with nutrition. This is done by 

 means of vessels which compose the vitelline circulation, but, im- 

 portant as this circulation is in the fowl's egg, it is of very brief 

 duration in the human subject, for the supply of nutritious 

 material is soon exhausted, probably at the sixth week. 



Placental or Fetal Circulation (Fig. 444). By the sixth week 

 the placenta is formed and the connection has been made by which 

 the embryo receives its nourishment from the maternal blood. 



4 5 



FIG. 443. Series of diagrams repre- 

 senting the relationship of the decidua 

 to the ovum at different periods. The 

 decidua are colored black, and the ovum 

 is shaded transversely. In 4 and 5 the 

 vascular processes of the chorion are 

 figured. 1, Ovum entering the con- 

 gested mucous membrane of the fundus 

 decidua serotina; 2, decidua reflexa 

 growing around the ovum ; 3, comple- 

 tion of the decidua around the ovum ; 

 4, general growth of villi of the 

 chorion ; 5, special growth of villi at 

 placental attachment, and atrophy of 

 the rest (copied from Dalton). 



