At the end of the cloacal period, the wolffian or mesonephric ducts guide Mullerian or paramesonephric ducts, which are newly formed from celomic epithelium at the cranial end of the genital ridges, toward the urogenital sinus derivatives of the cloaca. Approaching the perineum, the two paramesonephric ducts bend inward and fuse into a single tube in the midline. The distal tips, which have remained in direct contact with the lateral mesonephric ducts, then gain access to the sinus by using the orifices of the regressing mesonephric ducts. The original columnar epithelium of the distal part of the fused structure transforms into a broad stratified squamous epithelium that practically fills the developing vaginal lumen. Central degeneration of the epithelium creates the vaginal lumen. Discrepancy between the relatively small vaginal orifice and the greatly expanding vaginal lumen in combination with the movement toward the surface shapes the hymen.
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