The allantois is a membranous structure that forms during the early development of embryos in reptiles, birds, and mammals. It plays a crucial role in the exchange of gases and waste products between the embryo and the surrounding environment. In mammals, the allantois contributes to the formation of the umbilical cord and the placenta, which are essential for nutrient and oxygen transfer. In addition to its role in waste management, the allantois also stores nitrogenous waste, such as uric acid, in some species. This adaptation helps to prevent toxicity within the developing embryo. Overall, the allantois is vital for the proper growth and development of embryos in various species, including humans and birds.