(deutoplasm), a nutritive substance that accumulates in the ovum of animals and man in the form of granules or membranes that sometimes merge into a solid yolk mass (in insects, bony fish, and birds).
Yolk has been discovered in the ova of all animals and man, but its quantity and distribution vary considerably. In ova with a small amount of yolk, the yolk particles are distributed evenly throughout the cytoplasm (isolecithal ova). In ova with a large amount of yolk, the yolk particles are concentrated either in the vegetative part of the ovum (telolecithal ova) or in the central part of the cytoplasm, around the nucleus (centrolecithal ova). The type of egg cleavage depends on the quantity and distribution of the yolk. Chemically there are three principal varieties of yolk: protein, fat, and carbohydrate. However, in most animals the yolk granules have a complex chemical composition and contain proteins, fats, carbohydrates, ribonucleic acid, pigments, and mineral substances. For example, in the mature hen’s egg, the yolk contains 23 percent neutral fat, 16 percent protein, 11 percent phospholipids, 1.5 percent cholesterol, and 3 percent minerals. Various organelles of the ovum participate in the synthesis and accumulation of the yolk: the Golgi apparatus, endoplasmic reticulum, and mitochondria. In many animals the protein component of the yolk is synthesized outside the ovary and enters the growing ovum by means of pinocytosis. In some invertebrate animals the yolk may also accumulate in special cells of the ovary—the yolk cells, on which the developing embryo is nourished.
T. B. AIZENSHTADT