380 



THE ORGANS 



c — 



d — 



"C 



fibrin, and may form dense layers upon the surface of the chorion. 

 (Fig. 269.) 



The Placenta Uterina.^ — This develops from the decidua sero- 

 tina. The latter becomes much thinner than the rest of the decidua 

 (decidua vera), but still shows a division into a deeper spongy portion 

 containing gland tubules, and a superficial compact portion in which 

 are large numbers of decidual cells. From the superficial portion 



connective- tissue septa — placental 

 septa — grow into the foetal pla- 

 centa, as described above, separat- 

 ing its villi into cotyledons. Near 

 the margins of the placenta these 

 septa pass to the chorionic mem- 

 brane and form beneath it a thin 

 membrane, the subcJiorionic pla- 

 cental decidua. At the edge of the 

 placenta, where decidua serotina 

 passes over into the thicker decidua 

 vera, there is a close attachment of 

 the chorion to the former. 



As the placenta serves as the 

 place of interchange of materials 

 between the maternal and the foetal 

 circulations, the arrangement of the 

 placental blood-vessels is of espe- 

 cial importance. Arterial branches 

 from vessels of the uterine muscularis enter the serotina. In the 

 very tortuous course which these vessels take through the serotina 

 (Fig. 268) their walls lose their muscular and connective- tissue ele- 

 ments and become reduced to epithelial tubes. These branch in the 

 placental septa and finally open into the intervillous spaces along the 

 edges of the cotyledons. The veins take origin from these spaces 

 near the centres of the cotyledons. The maternal blood thus passes 

 through the intervillous spaces from periphery to centre, and in its 

 course comes into direct contact with the freely terminating chorionic 

 villi. It is to be noted that the blood-vessel systems of the mother 

 and of the foetus are both closed systems, and that consequently 

 there is no direct admixture of maternal and foetal blood. Inter- 

 change of niaterials must therefore always take place through the 

 capillary walls and through the walls of the chorionic vilh. (Fig. 268.) 



B 



Fig. 270. — Cross Sections of Human 

 Chorionic Villi at End of Pregnancy. 

 X250. (Schaper.) A, Small villus; B, 

 larger villus, a, Protoplasmic coat 

 (sj'ncytium); b, epithelial nucleus; c, 

 nuclear groups; d, small artery; e, small 

 vein; /, capillaries. 



