THE UTERUS DURING MENSTRUATION AND PREGNANCY 



239 



composed of a layer of trophectoderm with the outlines of its cuboidal cells 

 sharply defined (Fig. 248 A). This layer (of Langhans) forms and is 

 covered by a syncytium, the trophoderm. In the later months of preg- 

 nancy, as the villi 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 it continuous with^the 

 epithelium of the chorion laeve. 



Muscularis 



Uterine 

 artery Uterine veil, 



Uterine. 



artery in 



septum 



Decidua 



basalis 

 'ne artery in 

 decidual septum 



Intervillous 

 space 



Syncytium 



cal vein 



FIG. 247. Scheme of placental circulation (Kollmann). Arrows indicate supply and exhaust of 

 blood in the intervillous spaces. 



The Decidua Basalis. This, the maternal placenta, like the decidua 

 vera is differentiated into a compact layer, or basal plate, which forms the 

 floor of the intervillous spaces, and into a deep spongy layer (Figs. 246 

 and 247). 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 in the decidua vera. The glandular spaces are less 

 numerous in the spongy layer of the decidua basalis; between the spaces 



