The increasing bulk of the fluid serves to distend the fetal sac and surrounding uterus, and thus provides the fetus with room for growth and movement. It also prevents adhesions between the child’s skin and the amnion, which are a factor, when by mischance they do occur, in causing monstrosities and intrauterine amputations. The fluid with which it is surrounded keeps the fetus at an equable temperature in spite of variations of temperature in the mother’s environment, and minimizes the danger of injury to the fragile little body, from pressure or blows on the mother’s abdomen. And by acting as a water wedge, forced down by uterine contractions at the time of labor, it dilates the cervix sufficiently to permit the expulsion of the full term child. [Illustration: FIG. 22.—Placental blood vessels. Note their branching, tree-like arrangement. (Photographed from an injected specimen in the Obstetrical Laboratory, Johns Hopkins Hospital.) ] =The placenta.= The placenta, in lay parlance the after-birth, is really a thickened, amplified portion of the fetal sac, which has developed at the site of the implantation of the ovum. It is partly fetal and partly maternal in origin, being developed jointly from the chorion frondosum with its branching villi, and the underlying decidua basalis. The chorionic villi already referred to grow and branch in a tree-like fashion (Fig. 22), and push their way farther and farther into the uterine tissues creating the intervillous spaces which fill with maternal blood. From the time that the first fetal blood vessels appear in these floating villi, until the child is born, there is a constant exchange of nutriment and waste matter between the maternal and fetal blood; the arterial maternal blood in the intervillous spaces giving to the fetal blood in the villi the oxygen and other substances necessary to nourish and build the growing young body, and receiving in return the broken-down products of fetal activity. The waste is carried by the maternal blood stream to the mother’s lungs, kidneys and skin, by which it is excreted. This exchange of substances is accomplished by osmosis and also by selective powers of the cells in the villi. Thus the placenta virtually serves the fetus as lungs, stomach, intestines and kidneys throughout its uterine life.