

DEVELOPMENT OF THE FETAL MEMBRANES AND THE PLACENTA 61 



of the embryo. The chorionic villi are at first small and non-vascular, and consist 

 of trophoblast only, but they increase in size and ramify, while the mesoderm, 

 carrying branches of the umbilical vessels, grows into them, and in this way they 

 are vascularized. Blood is carried to the villi by the branches of the umbilical 



Placental villi imbedded in the 



f Decidua placentalis 



Uterine tube 



Allantois 



Umbilical cord 

 with its con- 

 tained vessels 



Non-placental villi im- 

 bedded in the decidua 

 capsularis 



Cavity of uterus 

 Yolk-sac 



Cavity of amnion 



Decidua vera 

 or parietalis 



' mucus in the 

 cervix uteri 



FIG. 34. Sectional plan of the gravid uterus ia the third and fourth month. (Modified from Wagner.) 



arteries, and after circulating through the capillaries of the villi, is returned to 

 the embryo by the umbilical veins. Until about the end of the second month 

 of pregnancy the villi cover the entire chorion, and are almost uniform in size 

 (Fig. 25), but after this they develop unequally. The greater part of the chorion 



Trophoblast 



esoderm 



Branches of umbilical vessels 

 FIG. 35. Transverse section of a chorionic villus. 



is in contact with the decidua capsularis (Fig. 34), and over this portion the villi, 

 with their contained vessels, undergo atrophy, so that by the fourth month scarcely 

 a trace of them is left, and hence this part of the chorion becomes smooth, and is 

 named the chorion Iseve ; as it takes no share in the formation of the placenta, it 



