The principles that comprise the cell theory are:
(i) All living organisms are composed of cells and products of the cells.
(ii) All cells arise from pre-existing cells.
In 1838, Matthias Schleiden, a botanist, did research on plants and discovered that all plants are made of cells.
At around the same time, Theodore Schwann, a zoologist , studied different types of animal cells and concluded that they are also composed of cells.
Schleiden and Schwann together gave the cell theory.
But a major drawback of this theory was that it could not explain how new cells are formed.
Rudolf Virchow, on careful analysis, observed that new cells are formed from pre-existing cells( Omnis cellula-e cellula).
Virchow gave the final shape to the cell theory based on the following points:
All living organisms are composed of cells and products of cells.
All cells arise from pre-existing cells.
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