Both the mesonephric duct and the ureter open into the common excretory duct. This again opens into the primary urogenital sinus. Gradually it is incorporated into the posterior wall of the primary urogenital sinus. The mesonephric duct and the ureter then open separately into the urogenital sinus. At S17, the openings of the mesonephric ducts define the boundary between the vesico-urethral canal and the definitive urogenital sinus. The openings of the ureter shift cranially and laterally as they develop. At S18, the openings of the mesonephric duct remain very close to each other and it is assumed that they migrate caudally. The triangular portion of the posterior wall of the bladder (trigonum vesicae), extending between the ureter and the mesonephric duct, is thought to originate from the mesonephric duct; therefore, it is of mesodermal origin. The remaining bladder is of endodermal origin.