As mentioned earlier, contact of the tip of the uterine canal with the pelvic part of the urogenital sinus produces a small elevation in the posterior of the sinus, which is called sinus tubercle. In the female fetus the endo-dermal epithelium of the urogenital sinus proliferates in the region of sinus tubercle to produce a pair of swellings called sinuvaginal bulbs, which extend from the urogenital sinus to the caudal end of the uterine canal. The sinuvaginal bulbs soon fuse together to form a solid block of endodermal tissue, which is called vaginal plate. The upper end of the vaginal plate is in contact with the inferior end of the uterine canal. Proliferation of cells in the vaginal plate leads to elongation of this plate from the 3rd to the 5th month. Subsequently, the vaginal plate becomes canalized by the degeneration and disappearance of the central cells of this plate. The origin of lining epithelium of vagina is a subject of controversy between different research workers. Some of the authorities hold the view that the vaginal epithelium is derived only from the peripheral cells of the vaginal plate and is, thus, entirely endodermal in origin. Other research workers, however, are of the opinion
that the lining epithelium of lower two-thirds of vagina is endodermal (being derived from the vaginal plate), whereas the lining epithelium of the upper one-third is mesodermal in origin and is derived from the lowermost part of the uterine canal (which they prefer to call uterovaginal canal).
Outer to the lining epithelium, the muscular and fibrous coats of vagina develop from the surrounding mesenchyme. An endodermal membrane, called hymen, temporarily separates the vaginal lumen from the cavity of the phallic part of the urogenital sinus (as described later, this part of the urogenital sinus forms the vestibule of vagina). The hymen usually becomes perforated shortly before or soon after birth.