Allantois

Allantois is an extraembryonic membrane found in amniotes. It forms early in development as an outgrowth of the caudal end of the yolk sac (hindgut) and expands into the surrounding extraembryonic coelom. In most species it develops a network of blood vessels that connects with the embryo through the body stalk, helping to support the growing embryo.

In birds and reptiles, the allantois grows to become a large sac that lies alongside the chorion. It fuses with the chorion to form the chorioallantoic membrane, which serves two main functions: gas exchange for the embryo and storage of nitrogenous wastes, primarily uric acid, released by the developing embryo.

In mammals, including humans, the allantois contributes to the vascular development of the placenta by forming part of the umbilical vessels and the connecting stalk. In many placental mammals the allantois is incorporated into the placental structure, helping to establish the blood supply between mother and fetus. In humans the allantois is greatly reduced; the portion near the bladder forms the urachus, a canal that connects the fetal bladder to the umbilicus, which later becomes the median umbilical ligament. The remaining vestiges of the allantois participate in the formation of the umbilical cord.

Overall, the allantois is one of the four traditional extraembryonic membranes (alongside the amnion, chorion, and yolk sac) and has species-specific roles in respiration, waste handling, and placental development.