1. A membranous sac that develops from the embryo of reptiles, birds, and mammals and functions in respiration and waste storage.
Usage: biology; embryology; technical

Examples

• The allantois is one of the four extraembryonic membranes in amniotic eggs.
• In mammals, the allantois contributes to the formation of the umbilical cord.
• The allantois allows the embryo to exchange gases through the eggshell in birds.
• During development, the allantois stores nitrogenous waste products from the embryo.
• The allantois is derived from the endoderm of the yolk sac.