This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though yolk sac is far more widely used.

Blood is conveyed to the wall of the yolk sac by the primitive aorta and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins to the tubular heart of the embryo.

As a rule, the vitelline duct undergoes complete obliteration by the 20th week as most of the yolk sac is incorporated into the developing gastrointestinal tract.

In the meantime Heuser's membrane, located on the opposite pole of the developing vesicle, starts its upward proliferation and meets the hypoblast.

The fish structure was innovated upon by the amniotes (reptiles, birds, and later mammals), which evolved bigger yolks and tougher eggs for life on land.