A temporary membrane formed during embryonic development in birds and mammals by the fusion of the chorion and allantois, which surrounds the embryo and aids in gas exchange and nutrient absorption.
Greek chorio- (membrane) + allantois (sausage-shaped structure), from the shape of the allantois which early anatomists thought resembled a sausage.
The chorioallantois is why chicken eggs are such perfect incubators—this membrane lets oxygen in and carbon dioxide out through the shell, creating a self-contained life support system that worked for millions of years before we invented technology!
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