- 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.
- a similar membrane in placental mammals that encloses a mostly hollow space and loses its nutritive function entirely as the placenta develops.
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
At the margins of the germinal area these layers are of course continuous with the rest of yolk-sac wall.
(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.
- the membranous sac that is attached to the ventral surface of the embryos of birds, reptiles, and some fishes and contains yolk
- the corresponding part in the embryo of mammals, which contains no yolk
- A membranous sac attached to an embryo, enclosing food yolk.umbilical vesicle
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
- 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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.