extra-embryonic membrane surrounding the embryo, amnion, and yolk sac; performs exchange of carbon dioxide from the embryo and oxygen from outside the shell, and collects wastes from the embryo; develops out of the gut of the embryo about 5-7 days into incubation, connected via blood vessels through the abdominal wall to the heart; about halfway through incubation it fuses with the chorion to form the chorioallantoic membrane