PREGNANCY 231 



ill t'oriii 111 (liferent iiiaiiiiiials, it iicxer imoKcs the entire outer 

 surface of the chorion. As the embryo and its membranes grow, 

 the amniotic sac and surrounding chorion covered with an over- 

 growth of uterine mucosa bulge into the uterine cavity. That 

 part or asjject which from the lieginning has been in closest relation 

 with the uterine wall will be the site of the formation of the placenta. 

 The placenta in humans is a disc-like plate, consisting of two com- 

 ponents, one chorionic and one derived from the uterine mucosa. 

 It is also termed the decidua basalis. The uterine mucosa which 

 grows over the chorion of the embryonic mass, where it fills the 

 uterine cavity, is known as the decidua capsularis and is continuous 

 with the decidua basalis, or placenta. The remaining uterine 

 lining also shows a mucosal thickening and is known as the decidua 

 vera. Branching tuft-like outgrowths from the chorion invade the 

 mucosa of the decidua capsularis and basalis. The chorionic villi 

 of the decidua basalis are much more elaborate, and it is here that 

 fetal nutrition, respiration, and excretion are effected. 



Chorionic F/Z/i. The epithelium covering of the chorion and its 

 villi is a syncytium, and from the underlying mesenchyme connec- 

 tive tissue develops carrying blood vessels. These villi become 

 much extended until only a very thin layer of epithelium separates 

 the fetal blood vessels from the blood in the sinuses of the uterine 

 mucosa into which the villi project. The old superficial uterine 

 wall epithelium disappears, although the superficial connective 

 tissue is still quite firm. The deeper portions of the chorionic villi 

 of the basalis lie in pools of blood in the deeper zone of the uterine 

 mucosa, where the blood has escaped from the broken vessels, as 

 described in the premenstrual condition of the uterus. The villi 

 are covered with a syncytial epithelium which becomes thinner 

 with greater ex^^ansion during embryonic growth. Within the 

 epithelial covering of each villus is a connective-tissue core, con- 

 taining two small arteries, veins, and capillaries. These vessels 

 connect with those in the umbilical cord which arise from the center 

 of the placenta opposite the surface associated with the uterine 

 mucosa. The umbilical cord has a jelly-like connective tissue 

 which encloses the two umbilical arteries, the umbilical vein, the 

 allantoic stalk, and the rudimentary yolk stalk. Nutritive material 

 and oxygen pass from the blood in the uterine ])ools through the 

 epithelial membrane of the chorionic villi into the capillaries, which 

 pass it through the umbilical vessels to the fetus. 



The fetus and its membranes grow rapidly until the uterine cavity 



