The<unk> is a structure in the development of the urinary and reproductive organs.
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 entodermal<unk>  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<unk> is, for a time, shut off from the anterior by a membrane, the<unk>  membrane, formed by the apposition of the ectoderm and entoderm, and reaching, at first, as far forward as the future umbilicus.
Behind the umbilicus, however, the mesoderm subsequently extends to form the lower part of the abdominal wall and symphysis pubis.
By the growth of the surrounding tissues the<unk>  membrane comes to lie at the bottom of a depression, which is lined by ectoderm and named the ectodermal<unk>
Cloaca 
Urachus
Splenic vein
Circle of Willis
Hepatic vein
Hepatic portal system
Abdominal wall
Azygos vein