with purified blood. By this means the ovum receives a rich supply of
nutriment, and as a natural consequence its growth is rapid.

Before very long the diameter of the ovum is greater than the depth
of the mucous membrane which surrounds it. Consequently that part of
the membrane which covers it is pushed into the uterine cavity, as
the ground is raised by a sprouting seed. Growth continues, the
bulging increases, and extensive alterations are wrought both in the
womb and in the capsule of the ovum. One of these alterations will be
more easily understood if we still think of the ovum as a seed, for
it grows away from its roots just as plants do. Most of the capsule,
therefore, is removed step by step farther from its source of
nourishment, for the maternal blood-vessels do not follow the
expanding sac but retain their original position at its base. Partly
on account of the lack of nutriment thus occasioned and partly on
account of the distention caused by the contents of the sac, atrophy
occurs in the distant portions of the sac's wall. As a final result
of these two factors, the maternal tissue which covers the ovum
becomes thinned and stretched; it is pushed entirely across the
uterine cavity; and by about the twentieth week meets the opposite
side of the cavity, to which it becomes adherent. Subsequently, the
sac which incloses the embryo becomes everywhere fastened to the
inner surface of the uterus and completely fills the uterine cavity.

THE AMNIOTIC FLUID.--The great enlargement of the uterus which is so
marked a characteristic of the latter part of pregnancy is due in a
measure to the luxuriant blood-supply, for better nutrition always
causes growth. In a far larger measure, however, it is due to
distention for which the product of conception is responsible. Beside
the fetus the inclosing sac also contains a considerable quantity of