This article is about the embryonic cloaca in placental mammals. For other uses, see Cloaca.
Tail end of human embryo thirty-two to thirty-three days old. Cloaca is visible at center left. The endodermal cloaca is labeled with green, while the ectodermal cloaca is seen as a colorless crest on the outside.
The hind-gut is at first prolonged backward into the body-stalk as the tube of the allantois; but, with the growth and flexure of the tail-end of the embryo, the body-stalk, with its contained allantoic tube, is carried forward to the ventral aspect of the body, and consequently a bend is formed at the junction of the hind-gut and allantois.
This bend becomes dilated into a pouch, which constitutes the endodermal cloaca; into its dorsal part the hind-gut opens, and from its ventral part the allantois passes forward.
The cloaca is, for a time, shut off from the anterior by a membrane, the cloacal membrane, formed by the apposition of the ectoderm and endoderm, and reaching, at first, as far forward as the future umbilicus.