Illi: r I I Kl S DURING MENSTRUATION AND PREGNANCY 235 



portions of the primitive trophectoderm. In the vessels of the chorionic 

 villi the chorionic circulation of the embryo is established. The blood 

 vessels of the uterus open into the intervillous blood spaces and here the 

 maternal blood circulates. The syncytial trophoderm covering the villi 

 is bathed in the maternal blood. Its functions are three-fold: (i) like 

 endothelium it prevents the coagulation of the maternal blood; (2) it 

 allows transudation between the blood of fetus and mother; and (3) it 

 assimilates substances from the maternal blood and transfers them to that 

 of the embryo. According to Mall (1915), the trophoderm also forms a 

 wall which dams or plugs the blood vessels as soon as eroded, and, with 

 the decidua (p. 240), permits but little blood to pass into the intervillous 

 spaces (cf. p. 241). 



fc^^^^gMl 



B 



FIG. 243. Human ova: A, of three weeks; B, of six weeks, showing formation of the chorion 

 lasve by degeneration of the chorionic villi (De Lee). 



The Chorion Laeve and Frondosum. The villi at first cover the 

 entire surface of the chorion. As the embryo grows more and more 

 out into the uterine cavity, the decidua capsularis and that portion of the 

 chorion attached to it are compressed, and the circulation in the inter- 

 villous spaces of these structures is cut off (Figs. 241 and 243). Thus, 

 beginning at the pole of the decidua capsularis, the villi in this portion 

 of the chorion degenerate during the fourth week and form the chorion 

 IcBve. The villi on that part of the chorion which is attached to the decidua 

 basalis continue their development, and, persisting, form the chorion 

 frondosum. This, with the decidua basalis of the uterus, constitutes the 

 placenta (Fig. 240) . The embryo is attached first to the chorion frondosum 

 by the body stalk (Figs. 77 B and 239), later by the umbilical cord (Fig. 



