The outer layer or membrane of a cell, also called the cell membrane or plasma membrane.
From Greek 'kytos' (cell) + 'derma' (skin), literally 'cell skin.' The term emerged in 19th-century cytology to describe the boundary layer that was observed surrounding cellular material under early microscopes.
Before we had electron microscopes, scientists could barely see the cell membrane itself, so they had to invent names based on what they theorized must exist—'cytioderma' is one of those older terms that scientists have mostly replaced with 'plasma membrane,' but it reveals how much 19th-century biologists were working with educated guesses!
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