| yolk sac n. A membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals and functioning as the circulatory system of the human embryo before internal circulation begins. |
| yolk sac 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. |
ThesaurusLegend: Synonyms Related Words Antonyms
| Noun | 1. |  yolk sac - membranous structure that functions as the circulatory system in mammalian embryos until the heart becomes functionalsac - a structure resembling a bag in an animal |
| 2. | yolk sac - membranous structure enclosing the yolk of eggs in birds, reptiles, marsupials, and some fishes; circulates nutrients to the developing embryo sac - a structure resembling a bag in an animal |
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