Before the urogenital sinus has opened to the exterior the mesenchym surrounding the urogenital depression exteriorly begins to thicken and produces an encircling elevation, the genital swelling. On the ventral side within this swelling appears a projection, the genital tubercle, which is thus surrounded by the genital swelling. The tubercle soon increases in size, so that the urogenital depression, now called the genital groove, becomes partly situated at its under aspect (Cut 5, Fig. 2). The lips of
this genital groove thicken and form the two genital folds. All these four organs are common in both sexes and represent the asexual or bisexual state of the external genital organs.
E. The Male External Genitals
In the male the genital tubercle increases enormously in size to form the penis. Its extremity becomes bulbously enlarged and forms the glans penis. The lips of the groove or rather of the vestibule, since by this time the urogenital membrane had broken through and had transformed the sinus urogenitalis into the vestibule, the so-called genital folds, meet together and fuse, thus converting the vestibule and the groove into the terminal portion of the male urethra and bringing it about, that the ductus ejaculatorii and the sinus pocularis open upon the floor of that passage. The prostate, consisting of several independent glands, has also its openings at this point. In its development the prostate belongs to the urethra as well as to the sinus urogenitalis. The two genital swellings are brought closer together in the male and form the scrotum, a sac containing two separate pouches into which the testes descend.
F. The Female External Genitals
In the female the vestibule, or the shallow depression which was formed through the breaking through of the urogenital membrane, remains open throughout life, and is termed the vestibule of the vulva. From the sides of the lower part of the sinus a pair of evaginations are formed and give rise to the Bartholinian glands. The vestibule being in fact the open sinus urogenitalis, the urethra and the vagina naturally have their orifices in the same.
The genital tubercle ceases to grow in the female and becomes the clitoris. The genital folds or the lips of the vestibule become prolonged and form the labia minora or the nymphae. The genital swelling increases in size through adipose and fibrous tissue. The part situated on the ventral side of the clitoris becomes the mons veneris, while the lateral parts are converted into the labia majora of the vulva.