embryonic cloaca

The embryonic cloaca is an endoderm-lined chamber that develops as a pouch-like dilation at the caudal end of the hindgut. Three sets of structures open into or bound it: the allantois joins it ventrally, the two mesonephric ducts enter laterally, and caudally the chamber ends blindly at the cloacal membrane. That membrane is formed by the union of proctodeal ectoderm, derived from the anal pit, with cloacal endoderm, and it is distinctive in having no intervening mesoderm between the two germ layers. The cloaca is thus the common terminal chamber into which the early hindgut and the developing urinary drainage converge before subsequent partition, and it is classified here within the urinary system on the strength of the mesonephric duct connection.