Updated by: Arabinda Naik
It is a sac like structure which arises from gut of the embryo near the yolk sac. In man the allantois is small and forms blood vessels of placenta and blood cells. It consists of outer mesoderm and inner endoderm.
(i) The cavity of the allantois serves as a urinary bladder. It stores the protein breakdown product in the form of water-insoluble crystals of uric acid and inside the egg up to the time of hatching.
(ii) The vascular “chorioallantoic membrane” lies in a close proximity to the inner surface of the porous shell. It acts as an extraembryonic lung by supplying the embryo with oxygen.