yolk sac

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A membranous sac attached to the embryo and enclosing the yolk in egg-laying vertebrates. In humans and other placental mammals, it functions as the circulatory system for the embryo before internal circulation begins.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy A membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional
• noun membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryo