The<unk>  is an embryological canal connecting the urinary bladder of the fetus with the allantois, a structure that contributes to the formation of the umbilical cord. The lumen (inside) of the<unk>  is normally obliterated during embryonic development, transforming the<unk>  into a solid cord, a functionless remnant. The<unk>  lies in the space of Retzius, between the transversalis fascia anteriorly and the peritoneum posteriorly.
Urachus
Cloaca 
Circle of Willis
Vitelline duct
Aortic arches
Thyroglossal duct
Mesonephros
Aortopulmonary Window