Fig. 9. Schematic representations of the descent of the Wolffian duct (WD) or vas deferens. Anterior view. At 6 weeks (A), the WD (red) opens to the urogenital sinus at a site adjacent to the ureteral orifice (UR; green). At 7–8 weeks (B, C), rather than descent, there are individual variations in the WD position along the mediolateral axis as well as in left/right difference in morphology of the urogenital sinus (URS). The future bladder and urethra are not discriminated in the sinus. At 8–9 weeks, the bilateral upper angles (arrows) of the URS start upward growth toward the umbilicus. At 9 weeks (D), depending on development of smooth muscles in the bladder as well as rhapdosphincter muscles of the urethra (RS), the descent of the vas deferens becomes evident. However, the epithelium is still same (arrowheads) between the future bladder and urethra. At 10–11 weeks (E), a drastic upward growth of bladder smooth muscles as well as a developing prostate (PR) accelerates the descent of the vas.