membrane forming a pouch containing fats, proteins, carbohydrates, minerals, vitamins, and water to nourish the growing embryo; membrane is formed in layers around the embryo and yolk in the ovary and the infundibulum section of the oviduct (see vitelline membrane); in its first few days the growing embryo pulls away from the yolk, but blood vessels in the membranes connect the large yolk sac through the abdominal wall to the embryo’s heart; the yolk sac shrinks as the embryo gets larger and is absorbed into the embryo’s body by the time of hatching