than 500 ova are required to insure regularity in the menstrual
function, it is clear that, if the surgeon finds it necessary to
remove one of the ovaries, the other will provide abundantly for
menstruation and for the bearing of children. Although every ovum
that will be produced as long as a woman lives has already sprung
into existence by the time she is born, not a single one ripens for
from twelve to fifteen years. The ripening process begins about the
time of puberty, and, unless suspended through the occurrence of
pregnancy, continues until the menopause. During this period, which
is also characterized by the periodical appearance of menstruation,
one ovum ripens each month; sometimes, though rarely, several ripen
at once, and this tendency is partly responsible for twins.

The human ovum is a tiny structure, measuring about 1/125 of an inch
in diameter. With the naked eye it can barely be seen; magnified by
the microscope it appears as a little round bag made of a transparent
membrane. Briefly described, the ovum is a single cell. That is, it
belongs to the simplest class of anatomical structures, and is one of
the millions upon millions of units that make up the body. It
contains a nucleus surrounded by nutritive material, the yolk. Yet
the quantity of yolk is exceedingly small. In this particular the
human ovum differs notably from the egg of birds, as it does also in
that it lacks a shell. Obviously, a shell would not only be useless
to an embryo developing within the body of its parent, but would shut
off the nourishment, which, since the ovum contains so little, must
necessarily be provided by the mother.

When the ovum has ripened, it becomes detached from the ovary, and
enters a fleshy tube about the size of a lead pencil, known as the
oviduct. There are two of these tubes, one running from the