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Avian egg yolk

The yolk mass, together with the ovum proper (after fertilization, the embryo) are enclosed by the vitelline membrane, whose structure is different from a cell membrane..The yolk is part of the cytoplasm of the egg, occupying more than 90% of it by the time of fertilization in the form of yolk granules. These are composed of lipids, proteins and enzymes, with maternal mRNA and glycogen surrounding them.