preparations for its reception. Within a few weeks the mucous
membrane becomes half an inch thick, that is, about ten times thicker
than it was; and all the elements entering into its composition,
become unusually active. The blood-vessels are congested; the glands
pour out a more elaborate secretion; and certain cells lay up a
bountiful store of material to be drawn upon in the formation of the
embryo and the building up of the structures that promote its
development.

The ovum is as likely to find a resting place at one spot as another
upon the surface of the uterine mucous membrane. The whole of that
surface has been made ready to receive it; yet the area actually
required to imbed the tiny object is extremely small. As the ovum
escapes from the oviduct and enters the womb, it is smaller, in all
probability, than the head of a pin. For at least a week after its
coming, diligent search is necessary to find the site of
implantation. Insignificant as it is at first, however, the region of
implantation later becomes very prominent, for it undergoes a
transformation that the rest of the mucous membrane does not share.
That is to say, it becomes the point of attachment of the Placenta,
an organ that has the very important function of drawing upon the
resources of the mother's blood. As the ovum sinks into this
especially prepared bed, the villi are formed. They break open the
adjacent capillaries of the mother, thus diverting her blood from its
accustomed course. The blood collects in microscopic lakes in contact
with the capsule of the ovum, and from them flows back into the
mother's veins. Through the veins it returns to her heart, by which
it is distributed through the arteries to the various regions of the
body. The tiny lakes, in which the villi hang, are thus made a part
of the mother's circulation and as such are regularly replenished