the first of four extraembryonic membranes of amniotes
to form during embryogenesis. Like the allantois, it arises
from the splanchnopleure (endoderm and splanchnic mesoderm)
to surround the mass of yolk in reptile and bird eggs.
It is connected to the midgut by the yolk stalk. The yolk
sac is also formed in mammals, despite the absence of