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,