Fig. 4. Horizontal sections of three embryos at 7 weeks before differentiation of urogenital sinus into the bladder and urethra. (A, B) A specimen of crown-rump length (CRL) 21 mm. (C, D) A specimen of CRL 22 mm. (E, F) A specimen of CRL 26.5 mm. Hematoxylin and eosin staining. The upper side of each panel corresponds to the posterior side of the pelvis. Panels (A), (C), and (E) display the plane 0.2 mm, 0.3 mm, or 0.4 mm superior to panels (B), (D), or (F), respectively. All panels were prepared at the same magnification. Scale bar in panel B=1 mm. (A, B) The Wolffian duct (WD) opening is located 0.2 mm inferior and less than 0.1 mm medial side of the UR. (C, D) The WD opening is located 0.3 mm inferior and 0.1 mm medial side of the ureteral opening (UR). (E, F) The WD opening is located 0.3–0.4 mm inferior and less than 0.1 mm medial side of the UR. Because of the inferiorly hanging course, the unilateral ureter is cut at multiple sites in panels (A), (C), and (E). LA, levator ani muscle; M, mesonephros; MD, Műllerian duct or paramesonephric duct; P, pubis; PC, peritoneal cavity; PX, pelvic autonomic nerve plexus; R, rectum; UA, umbilical artery; URS, urogenital sinus.