It contains the urachus, which is the obliterated form of the allantois. A urachal cyst is a sinus remaining from the allantois during embryogenesis. The vesico-urethral portion is the deepest, continuous with the allantois. The umbilical cord develops from and contains remnants of the yolk sac and allantois. A very small allantois is found in modern amphibians. A patent allantois can result in urachal cyst. The vesico-urethral portion is the deepest portion, continuous with the allantois. Germ cells migrate from near the allantois and colonize the primordial gonads. The other fetal membranes are the allantois and the secondary umbilical vesicle. Oxygen is absorbed by the allantois through the egg shell. An amnion, chorion, and allantois are present during embryonic life. Tbx4 has been shown to be expressed in the allantois, hindlimb, lung and proctodeum. The first to evolve was probably the allantois, a sack that develops from the gut/yolk-sack. In other words, the allantois takes no share in its formation.