of embryonic development and the means by which it is made possible;
no rational view of these matters could exist until the circulation
of the blood was described by William Harvey in 1628. After this
epoch-making revelation, it was accepted as true that the mother's
blood entered the unborn child and returned to her own system. But
that view eventually became untenable, for it was proved conclusively
that there is no communicating channel between the two. For years
after that, it was believed that before birth the womb manufactured
milk to sustain the child, just as the breasts do afterwards; but
this theory also was disproved; and, as I have said, only by the use
of the microscope have we learned the truth about fetal nutrition.

When thin slices of the placenta are magnified they are found to
contain countless numbers of tiny, finger-like processes; these are
the villi, and they constitute the major portion of the organ. The
villi seen in a mature placenta are the same as those which projected
from the capsule of the young ovum, but not these alone, for many
branches have sprouted from the original projections. The primary
trunks with all their branches hang from the capsule of the ovum and
extract nutriment from the mother's blood which surrounds them, just
as the roots of a tree extract it from the soil.

The interchange of material between mother and child as carried on in
the placenta can, perhaps, be made clearer if we compare one of the
trunks and its branching villi to a human forearm, hand, and fingers.
The hand, we will imagine, is held in a basin of water, in which, by
turning on a spigot and leaving the outflow unstopped, we have
arranged that the water changes constantly. In terms of this
illustration, the water corresponds to the mother's blood, rich in
oxygen, mineral matter, and all other kinds of essential nutriment;