THE UTERUS DURING MENSTRUATION AND PREGNANCY 



237 



pressed as the embryo grows (Fig. 241). To it is attached the chorion lave, the 

 villi of which degenerate. During the fourth month the increased size of the fetus 

 brings the capsularis into contact with the decidua vera with which it fuses, there- 

 by obliterating the uterine cavity. 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 (Fig. 241). 



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

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

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



Fig. 245. — Mature placenta, a, Entire organ, showing letai surface with membranes attached to the 

 periphery; 6, a portion of attached surface showing cotyledons (Heisler). 



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 tliick (Fig. 245) . Near 

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

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

 with, that of the chorion frondosum (Fig. 246). 



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

 branched tree-like structures which he in the intervillous spaces (Fig. 247) . The 

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

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

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



