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• YOLK SAC (noun) The noun YOLK SAC has 2 senses: 1. membranous structure that functions as the circulatory system in mammal embryos until the heart becomes functional 2. membranous structure enclosing the yolk of eggs in birds reptiles marsupials and some fishes; circulates nutrients to the developing embryo Familiarity information: YOLK SAC used as a noun is rare.
