Drawing from the book 'Microscopical Investigations' by Theodor Schwann (1810-1882), published in 1839. Schwann's most renowned work was on cell theory. He discovered the 'Schwann cells' and showed that an egg is a single cell until fertilised when it evolves into a complex organism. In 'Microscopical Investigations' he claimed that plants and animals are comprised of many cells, and that cells have a life of their own but this life is subordinated to that of the organism. The cell theory was pivotal in 19th century biomedicine.
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