From now on the cells which compose the mass rapidly increase in number and very shortly cease to be all of one kind. These different kinds of cells rearrange themselves and grow in such a manner that some of them begin to form the different parts of the baby’s body and others develop into two thin membranes that finally enclose the baby in a double sac. He is attached to the inner surface of the sac; the space which he does not occupy is filled with fluid and the sac itself is attached to the uterine lining at the point where the cell mass happened to stop and bury itself. This sac is what you have heard called the “bag of waters,” but the doctors refer to it as the =membranes=. As it enlarges and pushes out into the uterine cavity it still consists of two thin membranes except where it is attached to the uterus, at which point it grows into a thick, spongy mass of blood-vessels. These blood-vessels divide and branch out in a tree-like fashion and burrow into the uterine wall. As you will see later, it is through this mass of branching blood-vessels that the baby virtually eats and breathes and gives off waste materials during the nine months of his life within the uterus. The doctors refer to the mass as the =placenta= but you have heard it called the “afterbirth,” because it is expelled after the baby is born. [Illustration: FIG. 7.—Diagram showing the developing baby, at an early stage, with cord, membranes and placenta, within the uterine cavity. ] As the baby’s development advances the part by which he is connected with the placenta lengthens out into what is called the =umbilical cord=. There are blood-vessels in this cord through which blood constantly flows back and forth, carrying nourishment to the baby from his mother and waste matter from his little body to the placenta where it is taken up by her blood. But this exchange of materials takes place through thin membranes and consequently the blood of the mother and baby never mingle. Fig. 7 will give you an idea of how the sac of membranes, with the baby hanging inside, grows out into the uterine cavity; how at the point where the membranes are attached to the uterus the blood-vessels have developed into the thick, spongy placenta and how the baby is connected with it by means of the cord. In Fig. 8 you may see how the baby changes in appearance as the weeks of pregnancy go by. At the end of the fourth month he really looks quite like the baby that we are so eagerly preparing for. If we follow his development within the uterus month by month, we find that =by the end of the first lunar month, or fourth week=, the baby’s body is about ½ inch long and looks about as is suggested in the third little outline in Fig. 8. =At the end of the second month, or eighth week=, his head is fairly well shaped; bones are beginning to develop, webbed hands and feet are formed and the little body is about 1 inch long.