THE UTERUS DURING MENSTRUATION AND PREGNANCY 243 



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 of transfusion between the blood of fetus and mother; and (3) it assimi- 

 lates substances from the maternal blood and transfers them to that of the embryo. 

 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 com- 

 pressed, and the circulation in the intervillous spaces of these structures is cut off 

 (Figs. 234 and 236). Thus, beginning at the pole of the decidua capsularis, the 



FIG. 236. Human ova: A, of three weeks; B, of six weeks, showing formation of chorion laeve by 

 degeneration of the chorionic villi (De Lee's Obstetrics). 



villi in this portion of the chorion degenerate during the fourth week and form 

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

 decidua basalis continue their development and persisting form the chorion fron- 

 dosum. This, with the decidua basalis of the uterus, constitutes the placenta. 

 The embryo is attached first to the chorion frondosum by the body-stalk, later 

 by the umbilical cord (Fig. 234) . Through the umbilical vein and arteries in the 

 latter the placental circulation of the embryo takes place. 



The Decidua Vera. During the first phase of menstruation the uterine 

 mucosa begins to differentiate into a broad superficial compact layer and into a 

 narrower deep spongy layer in which are found the dilated ends of the uterine 

 glands. After pregnancy these two layers are still further differentiated in the 



