The cloaca is a distal expansion of the hindgut covered by the cloacal membrane. Between four and six weeks of embryonic life, marked differences in growth between the anterior and the posterior parts of the cloaca make the cloaca transform into a U-shaped tube, which then splits into the anterior urogenital sinus and the posterior anal canal after the cloacal membrane has disintegrated. The groove-like communication between the two parts is incorporated and will form a hairless median strip on the future clinical perineum. The deeper part of the urogenital sinus differentiates into the urethra and trigone of the bladder. The superficial part becomes the vestibule with its mantle tissue differentiated into erectile and fascial structures. Growth inhibition of most erectile structures and caudalward bending of the original phallic structure flattens the vulva with its small clitoris and prominent labia minora flanking a relatively large persisting urogenital orifice. In the absence of testosterone, the area between this orifice and the anus does not lengthen disproportionately as in the male and forms the clinical perineum. Bilateral concentrations of mesenchyme outside the mantle of the cloaca develop into the perineal muscles and labia majora.

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