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On 1595 he invented the first microscope.

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While observing cork through his microscope on 1665, Hooke saw tiny boxlike cavities, which he illustrated and described as cells. He had discovered plant cells! Hooke's discovery led to the understanding of cells as the smallest units of life—the foundation of cell theory.

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On 1668 he created his own microscope.
He called what he saw "animacules".

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In 1838 Schleiden defined the cell as the basic unit of plant structure.

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In 1838 Schwann defined the cell as the basic unit of animal structure.
In 1839 concluded that all tissues are made up of cells: this laid the foundations for the cell theory.

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Virchow's cellular theory was encapsulated in the epigram Omnis cellula e cellula ("all cells (come) from cells"), which he published in 1855.