yolk


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yolk

1. the substance in an animal ovum consisting of protein and fat that nourishes the developing embryo
2. a greasy substance secreted by the skin of a sheep and present in the fleece

Yolk

 

(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

yolk

[yōk]
(biochemistry)
Nutritive material stored in an ovum.
The yellow spherical mass of food material that makes up the central portion of the egg of a bird or reptile.
References in periodicals archive ?
The layers are the chalaziferous layer which is continuous with the chalazae which hold the yolk in place, the inner thin layer, the firm or thick layer and the outer thin layer.
An extender with a chemically defined protein base has been the target of several researches as there are disadvantages to using egg yolk or milk because they are organic products and they vary widely in their composition from one batch to another.
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Not just any laying hen can produce the golden yolk.
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Yolk Recruitment has been supported by an additional PS260,000 business finance grant from the Welsh Government for the creation of 29 new jobs by 2018.
First, separate the egg yolks from the egg whites, setting the whites aside.
In some chickens form various reasons it happened that, the yolk sac does not undergo absorption and remains unabsorbed in the abdominal cavity.
yolk height (mm)/yolk diameter (mm) x 100 Yolk ratio = yolk weight (g)/egg weight (g) x 100
Never seen a triple yolk but, in the early 1980s, I bought a dozen eggs from my milkman one Easter and every one was a double yolker.
However, a spokesman for the British Egg Information Service said: 'The double yolks tend to come from younger chickens whose hormone systems are not fully developed yet.