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yolk sac

noun, Embryology
1.
an extraembryonic membrane that encloses the yolk of eggs in birds, reptiles, and marsupials and that circulates nourishment from the yolk to the developing embryo.
2.
a similar membrane in placental mammals that encloses a mostly hollow space and loses its nutritive function entirely as the placenta develops.
Origin
1860-1865
1860-65
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for yolk-sac

yolk sac

noun (zoology)
1.
the membranous sac that is attached to the ventral surface of the embryos of birds, reptiles, and some fishes and contains yolk
2.
the corresponding part in the embryo of mammals, which contains no yolk
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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yolk-sac in Medicine

yolk sac n.
A membranous sac attached to an embryo, enclosing food yolk. Also called umbilical vesicle.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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yolk-sac in Science
yolk sac  
A sac that is attached to the gut of an embryo and encloses the yolk in bony fish, sharks, reptiles, mammals, and birds. In most mammals, the yolk sac functions as part of the embryo's circulatory system before the placenta develops.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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