From the earliest period of gestation, the middle membrane, called the chorion, (_g. g._ P. XII.) is covered, on its outer surface, with a number of small protuberances called _villosities_, which subsequently become true blood vessels. About the fourth month these have increased very much in size and number, and have all become conglomerated into one mass, in form like a mushroom. This is called the _Placenta_. It is almost entirely formed of blood vessels, which seem to attach themselves at one end, by open mouths, to the open mouths of other blood vessels on the inner walls of the uterus (_i. i._ Plate XII.) At the other end these vessels are drawn together and lengthened out into a long tube, called the _umbilical cord_, or _navel string_, which finally enters the body of the child at the navel and so establishes the connexion between it and the mother.--(_c._ Plate XII.) The blood vessels in the placenta, umbilicus, and fœtus, like those in the maternal body, are of two kinds, _Arteries_ and _veins_. The arteries, which come from the _left_ side of the heart, carry the pure blood, which contains all the materials for forming and nourishing every part of the system. The veins contain the blood in its impure state, and take it to the _right_ side of the heart, from whence it is forced into the lungs to be purified by the act of breathing. The blood is made impure by some of its constituents being absorbed, to form the different parts of the body, and by having thrown into it a quantity of waste and poisonous matter no longer needed. The course of the blood, therefore, is from the left side of the mother's heart along her arteries till it reaches the arteries of the uterus, from them it passes into those of the placenta, and thence into those of the umbilicus which convey it into the body of the child. When there it circulates in its arteries, supplies the material for its further increase and development, becomes in consequence impure, and passes into its veins, the same as in the maternal body. From these veins it passes into those of the umbilicus and placenta, and, apparently, into those of the mother, by which it is conveyed to the right side of her heart, and by its action to her lungs, to be again purified when she breathes. This explains what was previously stated, that the child uses the mother's heart, lungs, and stomach, while in the womb, and has, therefore, no occasion to use its own.