Yolk Sac

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Yolk sac

An extraembryonic membrane which extends through the umbilicus in vertebrates. In some elasmobranchs, birds, and reptiles, it is laden with yolk which serves as the nutritive source of embryonic development.

In mammals, as in birds, the yolk sac generally develops from extraembryonic splanchnopleure, and extends beneath the developing embryo. A blood vessel network develops in the mammalian yolk sac lining. Though these blood vessels are empty, they play an important role in absorbing nourishing food and oxygen from the mother. Thus, although the yolk sac in higher mammals may be considered an evolutionary vestige from its yolky-egged ancestors, it still serves important functions in the young embryo. As the embryo ages, the yolk sac shrinks in size, and the allantois takes over the role of nutrition. See Allantois

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Yolk Sac


the organ of nutrition and respiration in the embryos of cephalopod mollusks, cartilaginous and bony fish, reptiles, birds, mammals, and man.

The yolk sac arises in the early stages of embryonic development, usually by means of the overgrowing of the yolk with endoderm and with the visceral layer of the lateral plates, and consists of an enlarged outgrowth of the midguts, the cavity of which in the majority of animals (except higher mammals and man) is filled with unbroken yolk. In the wall of the yolk sac blood cells and blood vessels are formed, which provide for transport of nutritive substances to the embryo and for its respiration. As the embryo develops, the size of the yolk sac decreases, its cavity shrinks, and it is either gradually drawn into the body cavity and resorbed or is cast off.

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

yolk sac

[′yōk ‚sak]
A distended extraembryonic extension, heavy-laden with yolk, through the umbilicus of the midgut of the vertebrate embryo.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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Absence of intratubular germ cell neoplasia in testicular yolk sac tumors in children: a histochemical and immunohistochemical study.
On the basis of the presence of yolk sacs and size of the embryos, we determined that none of them were considered near-term embryos.
The yolk sac is a vascularized membrane similar to the placenta in mammals, which involves the yolk during embryonic development and is responsible for the transfer of nutrients to the embryo.
Three cases of yolk sac tumour were detected by morphological pattern.
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They removed a primary an 'intracardiac yolk sac germ cell tumour' from the heart of Aadhi Fabeer.
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For successful development in a hatchery, knowledge of the following ontogenetic events is essential: size and time at hatching, flexion of the notochord, duration of the yolk sac, the presence or absence of an adhesive organ, retinal pigmentation, opening of the mouth and intestinal lumen, development of fins, gas-bladder filling, and cutaneous pigmentation pattern (Santos & Godinho, 1994; 1996a; 2002).