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Development of the Genital Ducts
In both male and female embryos, two pairs of ducts develop—the mesonephric (wolffian) and parameso-nephric (müllerian) ducts. As with the gonad, these ducts passthrough an indifferent stage in which both pairs of ducts are pres-ent in both the male and the female embryo. Differentiation ofthe female ductal system is not dependent on development of the ovaries (Fig. 4.4).
In the male embryo, the mesonephric ducts, which drain the embryonic mesonephric kidneys, eventually form the epididymis, ductus deferens, and ejaculatory ducts. Inthe female embryo, the mesonephric ducts disappear. The para-mesonephric ducts persist to form major parts of the female reproductive tract (the fallopian tubes, uterus, and upper por-tion of the vagina). Paramesonephric ducts begin as invagi-nations of the epithelium covering the urogenital ridges, eventually forming longitudinally oriented tubes. The cranial end of each duct opens into the body (future peri-toneal) cavity. The ducts grow caudally until the two cau-dal ends contact the posterior wall of the urogenital sinus. This contact induces the posterior wall to proliferate and form the vaginal plate that eventually gives rise to the lower portion of the vagina. Meanwhile, the lower ends of the paramesonephric ducts fuse to form the upper portion of the vagina, cervix, and uterus. The cranial portion of each duct remains separated and forms the fallopian tube on each side. As the ducts move toward fusion in the mid-line, they carry a fold of peritoneum with them that becomes the broad ligament.
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