THE UTERUS DURING MENSTRUATION AND PREGNANCY 



233 



forms on the surface of the chorion solid cords of cells, the primary villi 

 (Fig. 239). The chorionic mesoderm grows out into these cords, which 

 branch profusely and become secondary, or true villi (Fig. 242). During 

 the development of the villi, the blood lacunae in the trophoderm around 

 the villi expand, run together, and produce intervillous blood spaces which 

 surround the villi and bathe the epithelium with blood. The syncytial 

 trophoderm, from being a spongy network, is now reduced to a continuous 



Uterine cavity 



Chorionic Cavity 



Chorion 



frondosum 



Decidua 



basalis 



Decidua capsularis 



Muscularis 



Decidua vera 



Cervical canal 



FIG. 240. Gravid uterus of about one month, longitudinal section. 



layer covering the outer surfaces of the villi and chorion. Branches of the 

 umbilical vessels develop in the mesoderm of the chorion and villi. The 

 mesodermal core of each villus and its branches is now covered by a two- 

 layered epithelium, an inner, ectodermal layer with distinctly outl ned 

 cuboidal cells, and an outer, syncytial trophoderm layer (Fig. 248 A). 

 The epithelium also forms solid columns of cells which anchor the ends of 

 certain villi to the maternal tissue. Islands, or nodes, of epithelial cells, 

 are attached to the villi or lie free in the decidua basalis; they represent 



