An embryonic structure that functions both as a respiratory membrane and as a site for the storage of nitrogenous wastes is the 1. amnion 2. yolk sac 3. allantois 4. chorion?

The correct answer is the 3. allantois. The allantois is an embryonic structure that serves both as a respiratory membrane, facilitating gas exchange, and as a storage site for nitrogenous wastes produced by the developing embryo. It plays a crucial role in the development of the urinary system and contributes to the formation of the umbilical cord in higher vertebrates.