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.

These projections, the so-called Villi, push their way into the
mucous membrane of the uterus and serve a two-fold purpose. One of
their functions is to fix the ovum in its new abode; and, though the
attachment is not at first very secure, it becomes stronger in the
course of time and is capable of withstanding whatever tendency the
activity of daily life may have to loosen it. The other, and equally
important, task of the villi, the majority of which dip into the
mother's blood, is to transmit substances to and from the embryo.

We have traced thus far the earliest steps in the development of the
ovum. One portion, we observed, was promptly set apart for the
construction of the future child; this favored portion became
inclosed by all the rest of the ovum, which has a more or less
spherical form and is technically called the fetal sac. The first
duty of the sac is to take root in the womb, and the second, no less
vital, is to draw nourishment from the mother. But neither of these
functions can be performed without the participation of the uterine