part of embryonic structure egg yolk AAO:0010579 BTO:0000371 Extraembryonic structure that serves as a nutrient store for the embryo during development.[AAO] MESH:A16.631.325 The majority of animals develop from a spherical egg with a single axis, the animal-vegetal (an-veg) axis. The animal half of the egg usually contains the nucleus of the oocyte, while the vegetal half of the egg is the preferred site for the storage of yolk. Eggs with an-veg polarity are considered ancestral for the vertebrates. Frogs, for example, have eggs with vegetally concentrated yolk and the nucleus located in the animal cytoplasm.[well established][VHOG] The yellow spheroidal mass of stored food that forms the inner portion of the egg of a bird or reptile and is surrounded by the white. UBERON:0007378 VHOG:0001742 ncithesaurus:Egg_Yolk nutrient store uberon yolk shelled egg