The
yolk sac is a membranous attached to an
embryo, formed by cells of the
hypoblast adjacent to the
embryonic disk. This is alternatively called the
umbilical vesicle by the
Terminologia Embryologica (TE), though
yolk sac is far more widely used. The yolk sac is important in early embryonic blood supply, and is incorporated into the primordial
gut during the fourth week of
development.