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 placentulÊ, 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. PLATE XIII. Pregnant uterus with cotyledons. Fig. 1. Uterus of the cow during pregnancy, laid open to show the cotyledons (_d_) on the internal surface of uterus (_c_). The ovary (_a_) is shown cut across, and the two halves are laid open to show the position of the discharged ovum at _a_'. Fig. 2 illustrates the relation of the fetal and maternal parts of a cotyledon. A portion of the uterus (_A_) is shown with the maternal cotyledon (_BB_) attached to it. The fetal portion (_D_) consists of a mass of very minute hairlike processes on the chorion (_E_), which fit into corresponding depressions or pits of the maternal portion. Each portion is abundantly supplied with blood vessels, so that a ready interchange of nutritive fluid may take place between mother and fetus. PLATE XIV. Vessels of umbilical cord. Fig. 1. Fetal calf with a portion of the wall of the abdominal cavity of the right side and the stomach and intestines removed to illustrate the nature of the umbilical or navel cord. It consists of a tube (1-1') into which pass the two umbilical arteries (3) carrying blood to the placenta in the uterus or womb and the umbilical vein (4) bringing the blood back and carrying it into the liver. The cord also contains the urachus (2') which carries urine from the bladder (2) through the cord. These vessels are all obliterated at birth. 5, liver; 5', lobe of same, known as the lobus Spigelii; 5'', gall bladder; 6, right kidney; 6', left kidney; 6'', ureters, or the tubes conducting the urine from the kidneys to the bladder; 7, rectum, where it has been severed in removing the intestines; 8, uterus of the fetus, cut off at the anterior extremity; 9, aorta; 10, posterior vena cava. (From _F¸rstenberg-Leisering, Anatomie und Physiologie des Rindes._)