urorectal septum
The ridge of mesoderm covered with endoderm that in the early developing embryo partitions the endodermal cloaca into the primary urogenital sinus (ventrally) and the anorectal canal of the hindgut (dorsally); the urorectal septum eventually fuses with the cloacal membrane, dividing it into a dorsal anal membrane and a larger ventral urogenital membrane - this area of fusion is represented in the adult by the perineal body[MP].