part_of starts_at ends_at urinary system cloaca EMAPA:27638 anatomical_structure The cloaca (CL) (TS:17-TS:20) is an epithelial lined cavity derived from endoderm, seen early in embryonic development and located at the posterior end of the embryo. The cloaca undergoes septation to form the urogenital sinus (ventral) and hindgut (dorsal). This common cavity is referred to as the cloaca until the urorectal septum makes contact with the skin at the surface of the embryo, and completely septates the urogenital sinus from the hindgut at TS:21. The cavity that remains, becomes part of the urogenital sinus and urethra. The cloaca is comprised of cloacal epithelium lining the lumen and the adjacent cloaca associated mesenchyme. Part of the cloacal epithelium forms the cloacal membrane. EMAPA:27573 TS17 TS20