The egg or ovum is simply a growing cyst, filled with a fluid, normally sterile, in which the developing embryo lives and swims. This fluid is the liquor amnii and it is retained by a cystic wall made up of two layers—the chorion, which represents the original cell membrane, and the amnion, which develops out of the fœtus. At maturity, the ovum will contain from one to two pints of liquor amnii. [Illustration: Fig. 15.—The chorionic villi about the third week of pregnancy. (Edgar.)] =The Liquor Amnii.=—The liquor amnii is of vast importance to the child. It allows free movement for the growing limbs and body, protects the child from sudden changes of temperature, prevents injury both from without and within, saves the child from birthmarks and deformities by keeping it from contact with the surrounding walls, and in labor lubricates the passages for the advancing part. In a measure, too, it probably serves as a food. In labor it forms a pouch called the _bag of waters_, which aids in dilating the os. [Illustration: Fig. 16.—Diagram illustrating relations of structures of the human uterus at the end of the seventh week of pregnancy. (American Text Book.)] Gradually, as nutrition becomes more abundant at the site of the growing placenta, a stalk-like structure thrusts out from the fœtal abdomen and forms an attachment with the formative placenta. This is called the ventral stalk and as soon as the communication with the placenta is established, it is combined with other parallel structures and becomes vascularized, to form the umbilical cord. [Illustration: Fig. 17.—Maternal surface of the placenta and membranes. The cord protrudes from the cavity which held the fœtus. (Edgar.)] =The Umbilical Cord.=—The umbilical cord at maturity measures from five to fifty inches in length and from one-half to one inch in thickness. The cord is composed of a gelatinous connective tissue, called Wharton’s jelly, in the midst of which lie the twisted vessels (two arteries and a vein) that supply the embryo with air and food and carry off the waste. =The Placenta.=—The placenta or “after-birth” is an oval or circular somewhat flattened disc, six to ten inches in diameter, and three-quarters to one and one-half inches thick. It weighs about a pound and a half. It is the organ of respiration and nutrition for the fœtus. [Illustration: Fig. 18.—Fœtal surface of human placenta. (Eden.)] [Illustration: Fig. 19.—The egg at term with uterus removed and child showing through the membranes. (Edgar.)]