Tissue in developing embryos that arises from both the endoderm (inner layer) and mesoderm (middle layer), or the cells that give rise to these layers.
From 'endo-' (inner), 'meso-' (middle), and 'derm' (skin/tissue layer). This embryological term emerged in the 19th century as scientists studied how organisms develop from fertilized eggs.
The fact that some embryonic tissues blur the line between endoderm and mesoderm shows that nature's categories aren't always perfectly separate—development is messier and more flexible than simple labels suggest.
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