It is plain that we have now in our hands the fetal sac, the development of which we have already traced from the beginning. The wall of the sac, it will be recalled, was originally of the same formation throughout; but when the ovum became imbedded in the womb, that part of its capsule which remained in permanent contact with the mother's blood underwent special development, whereas the rest of the capsule gradually pushed away from its primary position and, becoming stunted in its growth, even lost to some degree the development it had attained. This latter portion, the veil that passes from the edge of the placenta, is formed of the two membranes we have mentioned, namely, the chorion and the amnion. The placenta is, for the most part, a highly developed portion of the chorionic membrane, which became specialized simply because it happened to receive the best supply of blood. At the time of birth the placenta measures nearly an inch in thickness, is as large around as a breakfast-plate, and generally weighs a pound and a quarter, that is, approximately one-sixth of the weight of the child. This relation between the weight of the placenta and of the child is regularly maintained; therefore, the larger the child the larger the placenta associated with it. The placenta has two surfaces, easily distinguished from each other. The raw maternal surface was formerly attached to the inside of the uterus; the fetal surface, covered by the amniotic membrane, was in contact with the amniotic fluid. Across the fetal surface run a number of blood-vessels containing the child's blood, converging toward a central point at which the umbilical cord is inserted. The point at which the cord is attached affords the simplest means of distinguishing the two surfaces of the placenta. Our knowledge as to how the exchange of food and excretory products between mother and child is carried on by the placenta has been gained chiefly through the microscope. The oldest medical writings, as we might suppose, express very fanciful ideas regarding the nature of embryonic development and the means by which it is made possible; no rational view of these matters could exist until the circulation of the blood was described by William Harvey in 1628. After this epoch-making revelation, it was accepted as true that the mother's blood entered the unborn child and returned to her own system. But that view eventually became untenable, for it was proved conclusively that there is no communicating channel between the two. For years after that, it was believed that before birth the womb manufactured milk to sustain the child, just as the breasts do afterwards; but this theory also was disproved; and, as I have said, only by the use of the microscope have we learned the truth about fetal nutrition.