The period of yolk nutrition in the human is brief and probably variable. The minute size of the egg renders it impossible for it to furnish nutriment for any length of time, as is the case with the embryo of the fowl. From five to eight days after conception takes place, a membrane is formed around the ovum, called the chorion. Outside of this is still another membrane attaching itself to the womb. The internal surface of the chorion is supplied with villi or tufts resembling mulberry seed. Through these the embryo receives its nutrition, until at the close of the second month, from these tufts the placenta, or after-birth, begins to be developed. This is attached to some portion of the uterus, usually the upper lateral portion.
The Placenta is a spongy, vascular organ, at full term eight to ten inches in diameter, and two or three inches thick at center, thinning at the edges, weighing from three-fourths to one and one-fourth pounds. In appearance it is not unlike a piece of liver, only less solid.
It is the proper vascular apparatus serving the combined purpose of fetal nutrition, respiration and excretion. At least, through its absorption all these functions are accomplished.
This, with the membranes surrounding the fetus and umbilical cord, is called the after-birth.
The placenta (Plates VIII and IX) lies in complete juxtaposition with the uterus, with an almost imperceptible membrane interposed. The fibers and bloodvessels of the uterus and placenta do not interlace, as some suppose; each has a distinct set of bloodvessels and capillaries, and a separate circulation. Nutrition and excretion are carried on by exosmosis, or transudation through this very attenuated membrane.
The fetal circulation is an especially interesting phenomenon. Instead of the blood going to the lungs for oxygenation, the entire circuit is performed without this, the placenta serving the office of lungs as well as of the digestive organs.
From the placenta oxidized blood is brought through the umbilical vein, a large portion of it passing to the liver, but all eventually enters the heart by the ascending vena cava. By the Eustachian valve it is directed through the foramen ovale to the left auricle, from this to the left ventricle, which conveys it to the aorta.
Part of the blood, instead of taking this course, enters the right ventricle, and in place of going to the lungs through pulmonary arteries, passes at once to the aorta, through what is called the ductus arteriosus. After traveling the entire circuit, it is taken back to the placenta by two umbilical arteries, which are given off from the iliac arteries.
At birth the ductus arteriosus closes; the umbilical veins form the round ligament of the liver, and the umbilical arteries the round ligament of the uterus in the female, and the urachus, a ligament of the bladder, in the male.
The foramen ovale also closes, establishing a complete septum between the auricles of the heart.