Contribution to the study of the development of the terminal portion of the Wolffian duct and the ureter.

Ten human embryos ranging from 10 to 20 mm in vertex-coccyx length (Horizons XVI and XVII Streeter) aged between 33 +/- 1 and 39 +/- 1 days, were studied in order to interpret the mechanism which determines the displacement of the ureter from the dorsal part of the Wolffian duct toward the lateral wall of the bladder. We demonstrated that this movement occurs due to the fact that the duct common to the ureter and the Wolffian duct undergoes a process of internal rotation before it is absorbed by the urogenital sinus. This determines that the ureteral orifice enter the urogenital sinus laterally with respect to the Wolffian duct. This absorption indicates that the mucosa of the trigone is of mesoblastic origin and the remainder of the besical of endoblastic origin. The origin of the muscular apparatus of the detrusor is entirely mesoblastic.