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Soon after the cell and sperm have united, the cell divides into two. The process of dividing continues, each time doubling the number, and by the time that Drs. Rock and Hertig found their specimen, there was quite a mass of cells. At first there is a single layer of cells. This soon begins to infold, forming two layers, and then there is another infolding of the inner layer. Long chapters of books on embryology are devoted to describing the details of this process. It is sufficient for you to know that these three layers are called ectoderm, endoderm, and mesoderm – outer, inner, and middle layers. Infolding and twisting continue and soon definite shapes are evident.
From the ectoderm are formed the skin, hair, nails, the lining of the mouth and rectum, the nervous system and sense organs, such as the eyes and ears, or parts of them at any rate. The endoderm gives us the lining of the digestive tract, the liver and pancreas, the lungs, and some more. The mesoderm furnishes a great variety. It is enough to mention the bones and muscles and connective tissue, the circulatory system, and the organs of reproduction. In the fully developed body certain parts may have been formed from two or more of these layers.
But before the body has developed to any recognizable shape the outer layer of cells has been forming branches which push into the wall of the uterus like the roots of a tree. Thus the placenta, or afterbirth, is formed and from now on the tremendous demands of the embryo for nourishment are supplied by a sort of filtering, as it were, of materials from the mother’s blood to that of the child through these roots. The blood cells themselves are too large to cross this barrier as are other substances in the mother’s blood which are said to have too large molecules.
The embryo grows at an enormous rate. During its life in the womb it may increase its weight two billion times. Hence it is not remarkable that its need for nourishment cannot be met by fluid diffusing about the cells and that the heart and blood vessels begin to form about the third week.
The growth of the placenta keeps pace with this and from it membranes soon form. They fill with fluid which compresses the attachment between the child and the placenta until it becomes a narrow cord. The result is that we find the child suspended in the fluid retained by the membranes while it is connected to the mother only by the umbilical cord from its navel. It remains protected in this way until the time of its delivery.
Placenta is the Latin word for a flat cake and that accurately describes its shape after it has been delivered from the uterus. It is usually a good inch and a half thick at the middle, thinner at the edges. Its diameter is six or eight inches. It may be at any position on the wall of the uterus according to where the ovum happened to attach itself but fortunately this is usually near the top. If the ovum goes well to the bottom before taking hold, an unfortunate condition then occurs known as a placenta previa. This leads to a bad complication of pregnancy, for the placenta is over the external opening of the womb. When labor begins, this opening, called the os uteri, enlarges, pulling away from the placenta, which causes tearing of the blood vessels and bad bleeding. In normal labor after the baby has been delivered the uterus continues to contract, separating the placenta which quickly follows after the baby. Hence the very logical popular name, afterbirth. Then contraction of the uterus closes the vessels in the raw area left on its wall, and stops the bleeding.

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