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
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
That a factor from the head is required for maintenance of vitellogenesis is shown by Experiment 3, Table 2, in which decapitation midway through the reproductive cycle resulted in a 50% reduction in the percentage of yolky oocytes that remain endocytically active 18 hours later.
Left ovary condition was scored as: 1, smooth, entirely regressed, or juvenile; 2, slightly granular appearance; 3, small follicles apparent, but no follicular hierarchy evident; 4, follicles obvious, hierarchy evident; 5, large, yolky follicles evident; 6, yolky follicle ready for ovulation and/or egg in oviduct.
based on shared patterns of larval form and ecological conditions, McEdward and Janies (and others) have suggested that ancestral feeding larval forms give rise to various types of nonfeeding larval development by (a) loss of larval feeding structures and gain of large, yolky eggs, followed by (b) loss of planktonic dispersal, and then (c) gain of parental brood protection.
Marine biologists had predicted that, because plankton is scarce at great depths, animals there would be found to have large, yolky eggs.
Vitellogenesis is the production of yolky eggs in oviparous species, and involves transport of gene products from the liver to the ovary where proteins are deposited in the maturing oocytes.
Large invertebrate eggs are very yolky and have greater mass-specific energy content compared to eggs of planktotrophic species (Jaeckle, 1995: Moran and McAlister, 2009).
We will miss Thursdays and our yolky egg and soldiers.
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There was a collective gasp when the children saw the yolky splatter result from Mr.
Coventry cooks Dave Galvin and Kevita Bains were strong contenders with their dishes Bubble and Squeak with Hollandaise and Spicy Spanish Omelette, but Jamie Wynd, of Leicester, won the regional title with his signature dish, Black Pudding Yolky.