Egg Yolk

Egg yolk is the nutrient-bearing portion of an egg, distinct from the surrounding egg white, whose primary function is to supply food for the developing embryo. Reproductive systems that rely on a yolk for nourishment are termed lecithotrophic, in contrast to matrotrophic systems in which the mother supplies nutrients directly. In birds, most reptiles and insects the yolk is a specialized storage structure, while in some fish and invertebrates yolk material sits directly within the egg cell rather than forming a distinct organ.