 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.At a later stage the Wolffian duct and Müllerian duct open into its ventral portion.The cloaca is, for a time, shut off from the anterior by the cloacal membrane, formed by the apposition of the ectoderm and endoderm, and reaching, at first, as far forward as the future umbilicus.