an endoderm-lined outpouching of the hindgut (in humans, the yolk sac of a very young embryo) representing the primordium of the allantois; in most amniotes, it grows into the extraembryonic celom; in humans, the distal part of the allantoic lumen is rudimentary, not extending beyond the body stalk. Syn: allantoenteric diverticulum.