- opening of the primitive digestive cavity (archenteron) at one end of a gastrula.
- the first opening in the embryo.
- In many animals, the site of entry of extra-embryonic endodermal cells at the vegetal pole during gastrulation, as in amphibians and fishes.
- In amniotes, a transient pore at the embryonic-abembryonic boundary through which the primitive ectoderm and mesoderm are continuous, as in reptiles, birds, and mammals.