mother's blood underwent special development, whereas the rest of the
capsule gradually pushed away from its primary position and, becoming
stunted in its growth, even lost to some degree the development it
had attained. This latter portion, the veil that passes from the edge
of the placenta, is formed of the two membranes we have mentioned,
namely, the chorion and the amnion.

The placenta is, for the most part, a highly developed portion of the
chorionic membrane, which became specialized simply because it
happened to receive the best supply of blood. At the time of birth
the placenta measures nearly an inch in thickness, is as large around
as a breakfast-plate, and generally weighs a pound and a quarter,
that is, approximately one-sixth of the weight of the child. This
relation between the weight of the placenta and of the child is
regularly maintained; therefore, the larger the child the larger the
placenta associated with it.

The placenta has two surfaces, easily distinguished from each other.
The raw maternal surface was formerly attached to the inside of the
uterus; the fetal surface, covered by the amniotic membrane, was in
contact with the amniotic fluid. Across the fetal surface run a
number of blood-vessels containing the child's blood, converging
toward a central point at which the umbilical cord is inserted. The
point at which the cord is attached affords the simplest means of
distinguishing the two surfaces of the placenta.

Our knowledge as to how the exchange of food and excretory products
between mother and child is carried on by the placenta has been
gained chiefly through the microscope. The oldest medical writings,
as we might suppose, express very fanciful ideas regarding the nature