The inner, more fluid part of a cell's cytoplasm, typically containing most of the cell's organelles and nucleus.
From Greek endo- 'within' + plasma 'something molded or formed.' This term was coined in the late 1800s to distinguish the inner cytoplasm from the outer ectoplasm.
Under a microscope, single-celled organisms like amoebas show endoplasm as a clearer, grainier inner region—the contrast with the transparent outer layer was so striking that early biologists thought they had found two different substances.
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