extra-embryonic membrane surrounding the embryo, amnion, yolk sac, albumen, and allantois; performs exchange of carbon dioxide from the embryo and oxygen from outside the shell, and collects wastes from the embryo; develops along with the amnion out of the ectoderm and mesoderm (germ layers that develop into epidermis, muscles, blood vessels, and organs) in the gut area of the embryo and extends around the embryo, then (unlike the amnion) it doubles back to expand out around the yolk sac and albumen and attaches to the inner shell membrane; about halfway through incubation it fuses with the allantois to form the chorioallantoic membrane