an endoderm-lined outpouching of the hindgut representing the primordium of the allantois; in most amniotes, it grows into the extraembryonic celom; in humans it is an outpouching of the dorsal wall of the yolk sac; the distal part of the allantoic lumen is rudimentary, not extending beyond the body stalk; it becomes the urachus and remain as the median umbilical ligament. SYN: allantoenteric diverticulum.
