THE UTERUS DURING MENSTRUATION AND PREGNANCY 



245 



villi of which degenerate. With the increased size of the fetus, the capsularis 

 comes into contact with the decidua vera and fuses with it. Eventually it largely 

 degenerates, completely so opposite the internal os uteri, where the chorionic 

 villi are obliterated also. During pregnancy, the lumen of the cervix is closed by 

 a plug formed by the secretion of the glands opening into the cervix uteri. 



The Placenta. The placenta is composed of the decidua basalis, constituting 

 the maternal placenta, and of the chorion frondosum, the placenta fcetalis. The 

 area throughout which the villi of the chorion frondosum remain attached to 

 the decidua basalis is somewhat circular in form, so that at term the placenta is 

 disc-shaped, about seven inches in diameter and one inch thick (Fig. 238). Near 



FIG. 238. Mature placenta: a, entire organ, showing fetal surface with membranes attached to the 

 periphery; b, a portion of attached surface (Heisler). 



the middle of its fetal surface is attached the umbilical cord, and this surface is 

 formed by the amnion, the mesoderm of which is closely applied to and fused 

 with that of the chorion frondosum (Fig. 239). 



Chorion Frondosum. The villi of this portion of the chorion form profusely 

 branched tree-like structures which lie in the intervillous spaces (Fig. 240) . The 

 ends of some of the villi are attached to the wall of the decidua basalis and are 

 known as the anchoring villi. In the connective tissue core of each villus are 

 usually two arteries and two veins, branches of the umbilical vessels, cells like 

 lymphocytes and special cells of Hofbauer, the significance of which is not known. 

 Lymphatics are also present. The epithelium of the villi, as we have seen, is at 



