Cell theory has three tenets: cells are basic units of life, all life forms are composed of cells and cells come from pre-existing cells. Although there were numerous researchers who helped in development of cell theory over a very long time, the credit for its discovery is given to a number of scientists: Robert Hooke, Anton van Leeuwenhoek, Matthias Schleiden (was actually not credited by Schwann for his contribution), Theodor Schwann and Rudolf Virchow.
Robert Hooke is known for coining the term 'cell' and a microscope to see cells in wine cork. Leeuwenhoek made microscopes with high magnification and observed (& reported) a number of living cells (including blood cells and sperm cells). Schleiden and Schwann stated that animals and plants are composed of cells, which meant that cells are the smallest component of living beings.
Rudolf Virchow added the third tenet (cells come from pre-exiting cells) later on.
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