THE UTERUS DURING MENSTRUATION AND PREGNANCY 



23s 



covering the villi is bathed in the maternal blood. Its functions are three-fold: 

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

 allows 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 em- 

 bryo. 



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 decidual 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. 241 and 243). Thus, beginning at the pole of the decidua capsularis, the 



Fig. 243. — Human ova: A, of three weeks; B, of six weeks, showing formation of chorion laeve by 

 degeneration of the chorionic villi (De Lee). 



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 basaUs continue their development, and, persisting, form the chorion fro 

 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 (Figs. 

 77 B and 239), later by the umbilical cord (Fig. 241). Through the umbihcal 

 vein and arteries in the cord 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 



