Substance in an animal egg consisting of protein and fat that nourishes the developing embryo.

A membranous sac containing yolk attached to the ventral surface of embryos and early larvae in some fishes. Retained for a period post-hatching in those species which form yolk sac larvae.

Post-hatching larval stage occurring in many fish species whereby the yolk sac remains attached to the ventral surface for a period. The larva does not feed normally until the yolk is fully-absorbed.

Form of livebearing in which the larvae are mostly nourished by a yolk sac which is fully-absorbed prior to birth.