/ Thesaurus / Y Synonyms of yolk sac Find synonyms for: Noun1. yolk sac, vitelline sac, umbilical vesicle, vesicula umbilicus, sacusage: membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functional2. yolk sac, sacusage: membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryoWordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. Related Content Daily Word Quiz: immense Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language