yolk sac
yolk·sac
noun
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
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WordNet 2.1 (Princeton University). WordNet 2.1 Copyright Princeton University. All rights reserved.