As pregnancy advances and the fetal sac enlarges, the chorion læve covered by the decidua capsularis, or reflexa, is pushed farther out into the uterine cavity, until finally it quite reaches the opposite wall, meets the decidua vera and obliterates the entire space which had existed between the two membranes. This means that instead of a uterine cavity lined with decidua, and a tiny capsule somewhere off to the side lined with chorion, the latter has distended until it completely fills and really becomes the cavity within the uterine walls, thus lining the uterus with chorion and crowding the original lining out of existence. The decidudae capsularis and vera fuse in time and finally the capsularis degenerates and disappears. =The Amnion.= Returning for a moment to the blastodermal stage of the ovum, we find that the amnion, or inner membrane, first appears as a tiny vesicle over the dorsal surface of the embryo. Very soon, however, it invests the embryo completely, and the membranous sac is intact, excepting where it is pierced by the umbilical cord. The amnion, too, is derived in part from the ectoderm, but is a stronger, denser membrane than the chorion. At first there is an appreciable space, and some fluid, between the two membranes, but as the amnion increases in size with the advance of pregnancy, it comes in contact with and is loosely adherent to the chorion. Very early in its development the amniotic sac contains a pale yellow fluid known as the _amniotic fluid_, or _liquor amnii_, in which the fetus floats. This fluid increases in amount until the end of pregnancy and though the quantity is variable, it usually amounts to about a quart. The source of the liquor amnii is not definitely known, but it is generally believed to be of maternal origin, secreted from the amniotic membrane, though the possibility of its consisting partly of fetal urine cannot be overlooked. It is about 99% water, containing particles of dead skin and lanugo, a soft downy hair cast off from the body of the fetus, traces of albumen and both organic and inorganic salts. [Illustration: FIG. 21.—Diagram showing general structure and relation of membranes, placenta and cord. ] The amniotic fluid serves a variety of purposes. Since the intestines of the fetus contain lanugo and particles of dead skin, it is evident that the child swallows some of this fluid during its uterine life, and possibly obtains in this way much of the fluid necessary for its development.