Urogenital Sinus

Urogenital Sinus 

the cavity into which the efferent ducts of the reproductive and excretory systems empty in many vertebrates. The urogenital sinus opens into the cloaca in elasmobranchs and holocephalans, in chelonians, and in monotremes; in vertebrates that have a reduced cloaca—polypterids, chondrosteans, and holosteans, males of lower teleosts, and viviparous mammals—the urogenital sinus opens directly to the outside.

The urogenital sinus in the mammalian embryo develops from the lower ventral section of the cloaca; the gonaducts and urinary bladder open into this section, as do the ureters of monotremes. In female eutherians (placental mammals) and in women, the urogenital sinus is greatly shortened, forming the vestibule of the vagina. The vestibule is inferior to and separated from the vagina in ungulates, rodents, prosimians, anthropoid apes, and women by a membranous structure called the hymen. In males of viviparous mammals and in men, the upper portion of the urethra develops from the urogenital sinus.