THE UTERUS DURING MENSTRUATION AND PREGNANCY 



233 



chorion and villi. The mesodermal core of each villus and its branches is now 

 covered by a two-layered epithelium, an inner ectodermal layer w\th distinctly 

 outlined cubical cells, and an outer syncytial trophoderm layer (Fig. 242). The 



Decidua vera 



Border deft 



Border zone 



Apex of 

 decidua 



Cervical canal 



Posterior lip 

 of OS uteri' 



Fig. 240. — Section through a gravid uterus of twelve to fourteen days (KoIImann). 



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 and represent portions of the primitive troph- 

 ectoderm. In the vessels of the chorionic villi the chorionic circulation of the 



