Let us pause a moment, and reconsider the several steps in Nature's plan for nourishing the embryo and fetus. In the first place, as we have seen, there is the yolk sack or umbillical vesicle, filled with a fluid which nourishes the embryo. This gradually disappears in time, and is replaced by the "allantois" which by connection with the walls of the Uterus is enabled to nourish the fetus from and by the blood of the mother. For a short time, however, the embryo is nourished by both the yolk sack and the allantois. Then the allantois assumes the entire task, and the yolk sack passes away. Then, later, the placenta replaces the allantois, and the latter passes away as did its predecessor. The placenta works along the same general lines as the allantois, but is a far more complex way and with a much higher degree of efficiency, as we shall see presently. The placenta is connected with the body of the fetus by what is known as "the umbillical cord." The "umbillicus" or "navel" in the human being marks the place at which the umbillical cord entered the body of the fetus, from which it was severed after the birth of the child. The purpose of the umbillical cord is to contain and support the umbillical arteries and veins through which the fetus obtains nourishment from the placental substance, and through which the return blood flows. The rich red arterial blood is carried from the placenta to the fetus, and is then distributed over the body of the fetus, nourishing and building it up; the dark venous blood, laden with the waste products of the body of the fetus, is carried back to the placenta, there to be repurified and rendered again rich and nourishing. The story of the circulation of the blood of the fetus is most interesting. Although the fetal blood is derived from that of the mother, as we have said, yet the maternal blood does not pass directly from the circulatory system of the mother into that of the fetus; nor does the blood of the fetus return directly into the circulatory system of the mother. In fact, the fetal blood never comes in direct contact with that of the mother, or vice versa. The fetus has an independent circulatory system of its own, and yet, at the same time, from the moment of the placental connection until the moment of childbirth, all its nourishment is derived from its mother.