Observation before a general theory
In 1665 Hooke described compartments in cork as cells, observing walls remaining in dead tissue. The term supplied a vocabulary without establishing that plants and animals shared a fundamental organization. A general theory required comparisons among tissues and explanations of cellular formation, growth and contributions to living activity.
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Comparing plants and animals
Schleiden examined the cellular organization of plants in 1838, and Schwann extended the comparison to animals in 1839. Organisms with very different appearances could then be studied through common units. Early accounts nevertheless contained errors, including crystallization-like explanations of cell formation; recognizing the theory’s importance does not validate every original claim.
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Cells from existing cells
In the mid-nineteenth century Remak supplied evidence of cell division, while Virchow helped popularize the principle in pathology. Experimental observation and influential formulation are distinct contributions. Division connected growth and development with observable processes and corrected earlier suggestions that cells formed freely from noncellular material.
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Cells and levels of organization
A cell can constitute an organism or participate in tissues, organs and systems. Specialized functions do not erase shared cellular organization. Cell theory supplies a common framework without, by itself, explaining inheritance, metabolism or the origin of the earliest life. These questions required additional evidence and later molecular investigation.