1. A membranous sac that develops from the embryo of reptiles, birds, and mammals and functions in respiration and waste storage during development.
Usage: biology; embryology; also called allantois in singular form

Examples

• The allantoides is one of the four extraembryonic membranes found in amniotic eggs.
• In mammalian development, the allantoides contributes to the formation of the umbilical cord.
• The allantoides helps the embryo exchange gases and eliminate nitrogenous waste.
• During avian development, the allantoides fuses with the chorion to form the chorioallantois.
• Students studying embryology learn about the allantoides as part of fetal development.