The development of the caudal portion of the Müllerian ducts was studied in rats. The following results were obtained: (1) In the female embryonic rat the proximal thirds of the caudal parts of the Müllerian ducts meet to form the anlage of the biluminal cervix uteri of the uterus duplex. The middle and the distal thirds of the Müllerian ducts fuse and become the canalis vaginalis.(2) In the female embryonic rat the canalis vaginalis represents the anlage of the cranial part of the vagina.(3) During embryonic life the epithelium of the sinus urogenitalis proliferates and develops into the vaginal plate.(4) At birth the vagina is closed by the vaginal plate.9%0 In the male embryonic rat the caudal parts of the Müllerian ducts degenerate completely in their proximal and middle thirds. The distal third (canalis vaginalis) disintegrates cranially, caudally it gives rise to the prostatic vaginula.(6) In the male rat the prostatic vaginula is a remnant of the vaginal anlage, unlike the prostatic utriculus in man which contains a remnant of the uterine anl
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