THE UTERUS DURING MENSTRUATION AND PREGNANCY 



247 



grow, the trophectoderm is used up in forming the syncytium, so that at term the 

 trophoderm is the only continuous epithelial layer of the villi (Fig. 241 B). About 

 the margin of the placenta the trophectoderm persists as the closing ring, which is 

 continuous with the epithelium of the chorion laeve. Syncytial giant cells found 

 in the decidua basalis are said to be derived from the trophoderm of the villi, 



Sinus 



Muscidaris 



Uterine 

 artery Uterine vein 



Uterine. 



artery in 



septum 



Decidua 

 basalis 



Uterine artery in 

 decidual septum 



Intervillous 

 space 



Syncytium 



FIG. 240. Scheme of placental circulation (Kollmann's Handatlas). Arrows indicate supply and 

 exhaust of blood in the intervillous spaces. 



also a portion of the canalized fibrin found in the decidua basalis of the placenta 

 near term. 



Decidua Basalis. This, the maternal placenta, like the decidua vera is dif- 

 ferentiated into a compact layer or basal plate which forms the floor of the inter- 

 villous spaces, and into a deep spongy layer (Figs. 239 and 240). The first is the 

 remains of the compact layer of the uterine mucosa formed during the premenstrual 

 phase and partially destroyed by the implantation of the ovum. The second is the 

 modified spongy layer of the premenstrual period and, though thinner, shows the 

 same differentiation as does this same layer in the decidua vera. The glandular 

 spaces are less numerous in the spongy layer of the decidua basalis and between 



