• synonyms

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.
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Origin of yolk sac

First recorded in 1860–65
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for yolk sac

Historical Examples

  • At the margins of the germinal area these layers are of course continuous with the rest of yolk-sac wall.

    The Anatomy of the Human Peritoneum and Abdominal Cavity

    George. S. Huntington

  • (a) The vitello-intestinal or omphalo-mesenteric duct and the yolk-sac or umbilical vesicle.

  • The cleft in his breast might close up now, and the last remnant of his yolk-sac vanish forever.

    Forest Neighbors

    William Davenport Hulbert

  • The epiblast spreads uniformly over the yolk-sac and not on the one side only as in the former eggs.

  • In this thickened edge a circular vein arises, which brings back the blood from the yolk-sac to the embryo.

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
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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

yolk sac in Medicine

yolk sac

  1. A membranous sac attached to an embryo, enclosing food yolk.umbilical vesicle
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

yolk sac in Science

yolk sac

  1. 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.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.