Origin of Muellerian duct and its later development in relation to Wolffian duct and anogenital distance in the rat.

On day 14 of fetal life in the rat, Müllerian (paramesonephric) duct in both sexes was first observed close to Wolffian (mesonephric) duct. Its caudal end joined Wolffian duct at about the level of the middle of gonadal anlage. Thereafter, Müllerian duct became independent of Wolffian duct through its whole length. Therefore, the main part of Müllerian duct may be formed by budding from Wolffian duct. On day 16, the anogenital distance in male started to increase with thickening of Wolffian duct and disappearance of Müllerian duct. Castration of male fetuses on day 19 stopped later masculinizing development. Therefore, it is clear that the fetal testis is an organ crucial for the masculinization of the urogenital tract as it activates the secretion of masculinizing hormone(s).