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Text: A typical animal cell is 10 to 20 mm in diameter, or about five times small than the smallest visible particle. It was, therefore, not until good light microscopes became available, in the early part of the nineteenth century, that all plant and animal tissues were discovered to be aggregates of individual cells. This discovery, proposed as the cell doctrine by Schleiden and Schwann in 1838, marks the formal birth of cell biology. Each paragraph will be a test.

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