Fig. 1. Sagittal sections of two embryos before differentiation of urogenital sinus into the bladder and urethra. (A, B) A specimen of crown-rump length (CRL) 24 mm (7 weeks). (C–E) A specimen of CRL 35 mm (8 weeks). Hematoxylin and eosin staining. The left-hand side of each panel corresponds to the anterior side of the pelvis. Panel (A) (panel C) is the most medial side of panels (A) and (B) (panels C–E). Intervals between panels are 0.3 mm (A–B), 0.4 mm (C–D), and 0.2 mm (D–E), respectively. All panels were prepared at the same magnification. Scale bar in panel B=1 mm. (A, B) The Wolffian duct (WD) opening (arrow) is located less than 0.1 mm inferior and 0.3 mm medial side of the ureteral opening (arrowhead; UR). (D) The WD opening (arrow) is located 0.5 mm inferior and less than 0.1 mm medial side of the ureteral opening (arrowhead). (C) Smooth muscles start differentiation in the future bladder wall (stars), but the epithelium appears to be a common type of the urogenital sinus (URS). MD, Műllerian duct or paramesonephric duct; P, pubis; PC, peritoneal cavity; R, rectum; SMA, mesentery containing the superior mesenteric artery; ST, stomach; UA, umbilical artery; WD, Wolffian duct or mesonephric duct.