=Fetal Membranes.=—Inside of the decidual membranes we find the chorion and amnion. Whereas the deciduæ are maternal structures, the chorion and the amnion are derived from the ovum. The chorion is the more external of the two fetal membranes; it early surrounds the ovum and throws out processes (villi) toward the decidua reflexa and serotina. These processes are at first formed equally all over the chorion, but those which lie in connection with the decidua serotina grow more rapidly in size and complexity, and remain to form the main part of the placenta. The membrane in which the fetus lies is the amnion. Contained within the amnion is more or less fluid, the liquor amnii, in which the fetus rests as in a water-bed. It permits free movements on the part of the fetus and protects it from violence from the outside; it equalizes the pressure on the uterine walls, preventing pressure on the cord and placenta; and during labor it assists in the dilation of the os uteri and lubricates the maternal passages. From it also the fetus derives its supply of water. Occasionally water may form between the three membranes, and during labor the chorion may rupture and not the amnion, and the waters between them be discharged. In like manner, water which has formed between the decidua and the chorion may come away. These, then, are two sources of fluid which may be discharged from the vagina in labor when the amnion is intact. =Umbilical Cord.=—The organ by which the fetus is attached to the uterus is a whitish, glistening structure which springs from the fetus at the center of the abdomen, and passes to the internal aspect of the placenta. It contains two arteries and one vein surrounded by a gelatinous material (Wharton’s jelly), the whole being covered with a layer derived from the amnion. The vessels usually run tortuously in the cord, thus lessening the danger of compression, which might otherwise occur. This arrangement also tends to regulate and equalize the circulation of the blood through its vessels.