THE UTERUS DURING MENSTRUATION AND PREGNANCY 241 



the fibrin of the maternal blood and the chorionic trophoderm also partici- 

 pate (Mall, 191 5). From the basal plate, septa extend into the intervillous 

 spaces but do not unite with the chorion frondosum (Grosser) . Near term, 

 these constitute the septa placenta which incompletely divided the placenta 

 into lobules, or cotyledons (Figs. 245 and 247). The maternal arteries and 

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

 into the intervillous spaces (Fig. 247). Near their entrance they course 

 obliquely and lose all but their endothelial layers. The original openings 

 of the vessels into the intervillous spaces were formed during the im- 

 plantation 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 that varies in extent but never 

 circumscribes the placenta completely. This space is the marginal sinus 

 through which blood is carried away from the placenta by the maternal 

 veins (Fig. 249). The blood of the mother and fetus does not mix, 

 although the epithelial cells of the villi are instrumental in transferring 

 nutritive substances to the blood of the fetus and in eliminating wastes 

 from the fetal circulation into the maternal blood stream of the intervillous 

 spaces. 



Mall (1915) states that there is little evidence of an actual intervillous circulation; 

 the decidua and trophoderm are active in preventing this (pp. 239 and 240). Some 

 embryologists hold that the intervillous circulation is peculiar to the second half of 

 pregnancy. In summary, Mall regards the entire question as still open. 



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. 73 /./. During the first months of 

 pregnancy the embryo floats in the cavity of the amnion, attached to the 

 placenta by the umbilical cord (Fig. 241). Later, as we have seen, the 

 amnion fuses more or less completely to the chorion frondosum and laeve. 

 The decidua capsularis fuses with the decidua vera and largely disappears. 

 Before birth, the placenta is concave on its amniotic surface, its curvature 

 corresponding to that of the uterus (Fig. 249). 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 rup- 

 ture of the amnion and chorion lasve, 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 



