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A B C D E F G H I K L M N O P R S T U V W Y Z

The yolk sac is a pouch-like tissue in the early embryo that lies adjacent to the embryonic hypoblast portion of the developing inner cell mass. Its earliest stage during implantation in the mammal is derived from the blastocyst cavity and is termed the primary yolk sac. This is followed by the development of a secondary yolk sac, which develops within the primary yolk sac and displaces it; the secondary yolk sac is derived from the cells of the hypoblast. As development proceeds, the yolk sac is partially incorporated into the tissues of the intestines. Early in mammalian gestation and throughout gestation in other animals, the yolk sac serves as the reservoir for stored lipids, fats, and proteins, and it is also the site of early production of blood. Stem cells for both germ cells and hematopoietic cells reside within the yolk sac, close to the embryo, and migrate into the embryonic abdomen early in gestation. The yolk sac is resorbed into the embryo abdomen at varying times across species (e.g. it is still present at the time that hatching occurs in fish and amphibians and has been resorbed in birds, while in mammals, resorption takes place mid-gestation).