Fig. 3. Horizontal sections of two embryos at 6 weeks before differentiation of urogenital sinus into the bladder and urethra. (A–D) A specimen of crownrump length (CRL) 15 mm. (E–G) A specimen of CRL 17 mm. Hematoxylin and eosin staining. The upper side of each panel corresponds to the posterior side of the pelvis. Panel A (panel E) is the most superior side of panels (A–D) (panels E–G). Panel (B) is a higher magnification view of the central part of panel (A), panel (C) displays a plane 0.1 mm inferior side of panel (A) and 0.2 mm superior side of panel (D). Intervals between panels are 0.2 mm (E–F) and 0.1 mm (F–G), respectively. Scale bars=1 mm in panels A and D, 0.1 mm in panels B, C, and E–G. (A–D) The Wolffian duct (WD) opening is located 0.2–0.3 mm inferior and 0.1–0.2 mm medial side of the ureteral opening (UR). (F) The WD opening is located in the same supero-inferior level as and in 0.1 mm medial side of the ureteral opening. Because of the inferiorly hanging course, the unilateral ureter is cut at multiple sites in all panels. AO, aorta; FN, femoral nerve; K, definite kidney (metanephros); M, mesonephros; PC, peritoneal cavity; R, rectum; UA, umbilical artery; URS, urogenital sinus.