THE UTERUS DURING MENSTRUATION AND PREGNANCY 237 



and flattened until they resemble endothelial cells. At birth, or in case 

 of late abortion, the plane of separation is in the spongy layer. Only 

 the deep portions of the glands remain attached to the uterine wall, and, 

 by the division of their cells, regenerate the epithelium of the uterus. 



The Decidua Capsularis. The capsularis, as we have seen, becomes 

 thin as the embryo grows (Fig. 241). To it is attached the chorion losve, 

 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, thereby 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 secretions of the glands 

 opening into the cervix uteri (Fig. 241). 



\ 



FIG. 245. Mature placenta, a, Entire organ, showing fetal surface with membranes attached 

 to the periphery; b, a portion of attached surface showing cotyledons (Heisler). 



The Placenta. The placenta is composed of the decidua basalts, 



(etat)constituting the maternal portion, and of the chorion frondosum, the 



/contribution (Fig. 240) . 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. 245). Near 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. 246). 



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

 form profusely branched, tree-like structures which lie in the intervillous 



