THE UTERUS DURING MENSTRUATION AND PREGNANCY 249 



fibrin of the maternal blood. From the basal plate septa extend into the inter- 

 villous spaces but do not unite with the chorion frondosum (Grosser in Keibel 

 and Mall, vol. i, p. 162). Near term these constitute the septa placenta which 

 incompletely divide the placenta into lobules, or cotyledons (Fig. 240). The 

 maternal arteries and veins pass through the basal plate, taking a sinuous course 

 and opening into the intervillous spaces. Near their entrance they course ob- 

 liquely and lose all but their endothelial layers. The original openings of the 

 vessels into the intervillous spaces were formed during the implantation of the 

 ovum, when their walls were eroded by the invading trophoderm of the villi. 

 As the placenta increases in size the vessels grow larger. The ends of the villi are 

 frequently sucked into the veins and interfere with the placental circulation. 

 At the periphery of the placenta is an enlarged intervillous space, which varies in 

 extent and never more than partly surrounds the placenta. This space is the 

 marginal sinus through which blood is carried away from the placenta by the 

 maternal veins (Fig. 240). The blood of the mother and fetus does not mix, 

 but the epithelial cells of the villi are instrumental in transferring nutritive sub- 

 stances to the blood of the fetus, and in taking up excreta from the fetal circula- 

 tion and passing them into the maternal blood stream of the intervillous spaces. 

 The Relation of the Fetus to the Placenta and the Separation of the De- 

 cidual Membranes at Birth. — The relation of the embryo to the fetal membranes 

 has been described on p. 80. During the first months of pregnancy the embryo 

 floats in the cavity of the amnion attached to the placenta by the umbilical cord 

 (Fig. 234). Later, as we have seen, the amnion fuses more or less completely to 

 the chorion frondosum and laeve. The decidua capsularis largely disappears or 

 is fused to the decidua vera. Before birth, the placenta is concave on its amniotic 

 surface, its curvature corresponding to that of the uterus (Fig. 242). At term, 

 the duration of which is taken as ten lunar months, the muscular contractions of 

 the uterus, termed "pains, " bring about a dilation of the cervix uteri, the rupture 

 of the amnion and chorion laeve, and cause the extrusion of the child. With the 

 rupture of the membranes the amniotic liquor is expelled, the fetal membranes 

 remaining attached to the decidual membranes. The pains of labor begin the 

 detachment of the decidual membranes, the plane of their separation lying in 

 the spongy layer of the decidua basalis and decidua vera, where there are only thin- 

 walled partitions between the enlarged glands. Following the birth of the child, 

 the tension of the umbilical cord and the "after pains" which diminish the size 

 of the uterus, normally complete the separation of the decidual membranes from 

 the wall of the uterus. The uterine contractions serve also to diminish the size 



