The allantois is a membranous structure found in the embryos of reptiles, birds, and mammals. It plays a crucial role in the development of the embryo by facilitating gas exchange and waste removal. In some species, the allantois also contributes to the formation of the umbilical cord, connecting the embryo to the placenta. In reptiles and birds, the allantois stores waste products and helps in respiration, allowing the embryo to survive in an egg environment. In mammals, it is less prominent but still important for early development, as it aids in the transfer of nutrients and oxygen from the mother to the embryo through the placenta.