The allantois is an extension of the posterior wall of the yolk sac. Its exact role in development is unclear, though its blood vessels do become the umbilical blood vessels. It gives rise to a structure called the urachus which contributes to the superior wall of the urinary bladder. The distal part of the urachus becomes a fibrotic cord after birth and persists in the body as the median umbilical ligament. The median umblical ligament is located in the anterior abdominal wall and stretches between the bladder and the umbilicus.