be or in its canal; sometimes possibly in the womb.
On some occasions a squad of spermatozoids advances to meet the
descending egg, and numerous spermatozoids are often found in the
tubes, even as far as the abdominal cavity.

=Fixation of the egg. Formation of the Decidua.=--After fecundation,
the egg becomes attached to the mucous membrane of the cavity of the
womb. This mucous membrane proliferates and becomes gradually detached
from the womb to form the _membrana decidua_ which envelops the egg
or ovule. An egg fecundated and fixed in this way may keep its
position and grow during the first weeks of pregnancy, by the aid of
villosities covering its envelope which penetrate the wall of the
womb.

[Illustration: FIG. 18. Diagrammatic section in median plane of
the female genital organs. It shows the position of an ovule
which has just been discharged lying in the opening of the right
tube, and that of another ovary fecundated and surrounded by the
decidual membrane. In reality this could hardly coexist with the
other ovule freely discharged. In the right ovary are seen ovules
in various degrees of maturity in their Graafian follicles: also
a corpus luteum--an empty Graafian follicle after expulsion of
the ovule. The figure also shows the end of the penis in the
vagina at the moment of ejaculation of semen, and the position of
a preventive to avoid fecundation.]

[Illustration: FIG. 19. The mouth of the tube applied to the
ovary at the moment of expulsion of the ovule.]

=The womb. The placenta.= The womb or uterus is the size of a small
egg flattened in one direction. It terminates below in the neck or
_cervix_, which is prolonged into the vagina as a projection, called
the vaginal portion of the uterus. The cavity of the womb is continued
into the neck and opens below in the vagina by an aperture which is
round in virgins and is called the external _os uteri_. The walls of
the womb consist of a thick layer of unstriped muscle. When childbirth
takes place it causes tearing which makes the external os uteri
irregular and fissured. During copulation the aperture of the penis or
male organ is placed nearly opposite the os uteri, which facilitates
the entrance of spermatozoa into the uterus. (For the illustration of
these points see Fig. 18.)

The vitellus and the membrane of the egg enlarge with the embryo and
absorb by endosmosis the nutritive matter necessary for the latter,