PLATE XII. Fetal calf within its membranes (at mid term). The uterus is opened on the left side. In the uterus the fetus is surrounded by several membranes which are known as the amnion or inner layer, the allantois or central layer, and the chorion or outer layer. The amnion is nearest the fetus and forms a closed sac around it filled with a fluid known as liquor amnii, in which the fetus floats. The allantois is composed of two layers, which form a closed sac in connection with the urachus, or the tube which extends from the fetal bladder through the umbilical cord. The one layer of the allantois is spread over the outer surface of the amnion and the other over the inner surface of chorion. The allantois also contains a fluid which is known as the allantoid liquid. The chorion is the outer envelope or membrane of the fetus, completely inclosing the fetus with its other membranes. On the outer surface of this membrane are found the fetal placentulæ, or cotyledons, which, through their attachment to the maternal cotyledons, furnish the fetus with the means of sustaining life. The relation of the fetal and maternal cotyledons to each other is illustrated on the following plate.