membrane emerging as a sac from the hindgut's ventral wall; formed
from the splanchnopleure (combination of endoderm and splanchnic mesoderm).
Found in amniotes, it is one of the four extraembryonic membranes
(chorion, amnion, allantois and yolk sac) that are adaptations of
the terrestrial egg. It collects waste materials from the embryo,
and as a part of the chorio-allantoic membrane can be a site of gas