After its entrance into the cavity of the uterus prepared to receive and protect it, the mass of cells sinks into the soft, velvety lining of the organ. Here it is entirely surrounded by tissue which belongs to the mother. But just before implantation takes place the architecture of the ovum is modified in such a way as to indicate the trend of its subsequent development. We left it, a hollow ball passing down the oviduct; had we examined the sphere more closely we should have found its wall composed of a single layer of cells. At one spot, however, the wall soon thickens. The thickening is due to a specialized group of cells which gradually grows toward the hollow center of the ball. A little later, if we study the structure as a whole, we find it a small, distended sac, from the inner surface of which hangs a tiny clump of tissue. The clump of cells within and the inclosing sac as well are both requisite to the ultimate object of pregnancy; yet they fulfill very different purposes. The clump within will mold itself into the embryo; the inclosing sac will make possible the continued existence and growth of the embryo by securing and conveying to it nourishment according to its needs. These two structures, which from now on constitute the ovum, can best be considered separately and in the order of their development. We shall therefore first study the sac and in the next chapter the embryo. For a time after this sac, or ball, as you may choose to think of it, becomes implanted in the uterus, every part of its wall shares in the responsibility of procuring nourishment for the embryo. On this account the wall, or capsule, is for several weeks the most conspicuous part of the ovum. Its position is naturally advantageous, for, since it forms the outermost region of the structure and comes into immediate contact with the tissues of the mother, it has the first opportunity to seize and appropriate nutriment. Consequently, while there is still relatively little development in the embryo, the capsule of the ovum gives evidence of rapid extension; the wall becomes thicker, and the circumference of the sac increases. The significant thing about this growth, however, is the fact that it does not progress evenly. At some points cell-division is more active than at others, with the result that the surface of the ovum speedily loses its smooth, regular outline. Projections from the capsule appear; they increase in number and in length; and by the end of four weeks the ovum, as yet less than an inch in diameter, resembles a miniature chestnut-burr. To make the comparison more accurate, we must imagine such a burr covered with limp threads instead of rigid spines.