[yohk, yohlk]
the yellow and principal substance of an egg, as distinguished from the white.
Embryology. the part of the contents of the egg of an animal that enters directly into the formation of the embryo, together with any material that nourishes the embryo during its formation.
the essential part; the inner core.
a natural grease exuded from the skin of sheep.
Also, yoke.

before 1000; Middle English yolke, yelke, Old English geoloca, derivative of geolu yellow

yolked, adjective
yolkless, adjective
yolky, adjective

yoke, yolk.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
yolk (jəʊk)
1.  the substance in an animal ovum consisting of protein and fat that nourishes the developing embryoRelated: vitelline
2.  a greasy substance secreted by the skin of a sheep and present in the fleece
Related: vitelline
[Old English geoloca, from geoluyellow]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

O.E. geolca, geoloca "yolk," lit. "the yellow part," from geolu "yellow" (see yellow). Formerly also spelled yelk.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

yolk (yōk)
The portion of the egg of an animal that consists of protein and fat from which the early embryo gets its main nourishment and of protoplasmic substances from which the embryo develops.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
yolk   (yōk)  Pronunciation Key 
The yellow internal part of the egg of a bird or reptile. The yolk is surrounded by the albumen and supplies food to the developing young.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica


the nutritive material of an egg, used as food by a developing, embryonic animal. Eggs with relatively little, uniformly distributed yolk are termed isolecithal. This condition occurs in invertebrates and in all but the lowest mammals. Eggs with abundant yolk concentrated in one hemisphere of the egg are termed telolecithal. This occurs in many invertebrates and in all vertebrates lower than marsupial mammals. In arthropods, the yolk is massed near the centre of the egg; such eggs are termed centrolecithal

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
The cost of a lifetime spent opening eggs incompetently is a small amount of
  wasted time and some spilled yolk.
The raw egg starts to spin again because it has liquid yolk and egg white
Inside the pouch, the tadpoles live on the yolk leftover from their hatching.
Scavengers such as crickets might have fed on the yolk, and spiders later dined
  on the scavengers.
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