When thin slices of the placenta are magnified they are found to contain countless numbers of tiny, finger-like processes; these are the villi, and they constitute the major portion of the organ. The villi seen in a mature placenta are the same as those which projected from the capsule of the young ovum, but not these alone, for many branches have sprouted from the original projections. The primary trunks with all their branches hang from the capsule of the ovum and extract nutriment from the mother's blood which surrounds them, just as the roots of a tree extract it from the soil. The interchange of material between mother and child as carried on in the placenta can, perhaps, be made clearer if we compare one of the trunks and its branching villi to a human forearm, hand, and fingers. The hand, we will imagine, is held in a basin of water, in which, by turning on a spigot and leaving the outflow unstopped, we have arranged that the water changes constantly. In terms of this illustration, the water corresponds to the mother's blood, rich in oxygen, mineral matter, and all other kinds of essential nutriment; and the fingers are the villi. The blood-vessels in the fingers, to go a step farther, represent the blood-vessels which exist within the villi, connecting with the umbilical cord, and passing by that route to the body of the child. The blood which thus circulates through the villi, it is important to emphasize, is the child's blood; it cannot escape through the coating of the villi, just as our blood cannot escape through the skin of the fingers. Similarly, the mother's blood cannot enter the child; the two circulations are absolutely separate and distinct. It must be noticed, moreover, that the maternal blood not only brings to the surface of the villi everything the child needs, but it also takes away the waste products of fetal life. Let us select one of the foodstuffs necessary for the unborn child, and follow its course so far as it relates to fetal nutrition. The mother's blood brings sugar, for example, from her intestinal tract to the surface of the villi; through the coating of the villi the sugar passes into the fetal blood, is carried to the fetal heart, and distributed to the various fetal organs. They burn it, deriving heat and energy, and in return give off waste products, namely, carbonic acid gas and water, which are taken up by the fetal blood, borne back to the placenta, and pass again through the coating of the villi into the mother's circulation. These waste products are then transported to the mother's lungs and to her kidneys, and are finally thrown off from her body. Before the child is born, therefore, the placenta, which is an aggregation of villi, acts as its stomach, intestines, lungs, and kidneys.