THE UTERUS DURING MENSTRUATION AND PREGNANCY 



239 



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. 248 B). About 

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

 continuous with the epithelium of the chorion laeve. 



Decidua Basalts.— This, the maternal placenta, Uke 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. 246 and 247). The first is the 



Musctilaris 



Sinus 



Uterine 

 artery 



Uterine vein 



Uterine. 



artery in 



septum 



Decidua 

 basalis 

 Uterine artery in 

 decidual septum 



Intervillous 



space 



Syncytium 



Fig. 247. — Scheme of placental circulation (Kollmann). Arrows indicate supply and exhaust of blood 



in the intervillous spaces. 



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; between 

 the spaces occur syncytial giant cells said to be derived from the trophoderm of 

 the villi. It is in the plane of this spongy layer that the separation of the placenta 

 takes place at birth. 



