lation of semen as well as to erection.

We have seen above that the male and female germinal glands arise from
the same primitive organ in the embryo. If the embryo becomes male,
this organ is transformed into the two testicles which descend
gradually in the canal of the groin and become placed in the scrotum.
If it becomes female, the two sexual glands remain in the abdominal
cavity and are transformed into ovaries.

=The Genital Organs of Woman.=--The organs described in Chapter II
(Figs. 18 and 19), constitute the internal and more important part of
the female sexual apparatus. In women, the urethra opens externally on
its own account. It is much shorter and wider than in men. At its
external extremity is a small cavernous body called the _clitoris_,
which corresponds embryologically to the penis in man, and chiefly to
the glans. Like the latter it is specialized for sexual irritation and
possesses very sensitive nerves. The opening of the female urethra is
situated in front of the vulva directly under the pubic bone, at the
same place as the root of the male penis. From this point, on each
side of the middle line, extend two longitudinal folds, one external
covered with skin and called the larger lip of the vulva (Fig. 18,
_labia majora_), the other internal, hidden under the first, called
the lesser lip of the vulva (_labia minora_), and covered with thin
mucous membrane. Between the two lesser lips is the sexual aperture,
which, with the labia majora and minora is called the _vulva_. This
opening is distinct from that of the urethra, and leads to the
internal cavity or _vagina_ (Fig. 18). The vagina is about ten to
twelve centimeters long (2 to 2-1/2 inches) and terminates in a
_cul-de-sac_ which surrounds the vaginal portion of the womb, of which
we have spoken above.

In virgins the entrance to the vagina is more or less closed by a
delicate transverse membrane called the _hymen_, which is only
perforated by a narrow opening. At the first coitus the hymen is torn,
causing a certain amount of pain and bleeding. The walls of the vagina
are thrown into transverse folds, which render them somewhat rough.
The remains of the hymen torn by the first coitus afterward form
behind the vulva small excrescences named _carunculae myrtiformes_.

In the first chapter we have spoken of the changes undergone by the
fecundated ovule till it becomes the embryo and then the infant. It
remains to speak of the mechanism of expul