The innermost membrane that surrounds and protects a developing fetus, creating a fluid-filled sac.
From Greek amnion, possibly from amnos (lamb) or from a root meaning 'to wrap.' The term became standard in embryology starting in the 17th-18th centuries.
The amnion is so protective that when babies are born 'with the caul,' that's a piece of amnion that didn't tear—in medieval Europe, this was considered a lucky charm that people would sell.
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