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While observing a cork slice using a compound microscope, an English scientist Robert Hooke came across a honeycomb-like structure. These structures were actually cell walls of the cork's dead tissue. Thus, Hooke coined the term "cell" for the structures he observed.
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A Dutch biologist Anton van Leeuwenhoek observed the cells of a pond water with a microscope that he made.
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Anton van Leeuwenhoek made a number of furthered discoveries with his microscope. He published a letter to the Royal Society, including detailed illustrations of what he saw on a microscopic level. Among these were the first bacteria and protozoa.
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While observing plant cells, an English botanist Robert Brown discovered the center of the cell called the Nucleus.
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A German botanist Matthias Jakob Schleiden proposes that all plant tissues are made of cells. Proposing that cells are the basic building structure of all plants. This was the first ever generalization made about cells.
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A German botanist Theodor Schwann proposes that besides plants, animal tissues are composed of cells as well. This allowed the debates that plants and animals were fundamentally different in structure to reach a consensus. He pulled together theories that 1. All organisms consist of one are more cells. 2. Cells are the basic unit of structure for all organisms.
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A Swiss anatomist called Albrecht von Roelliker makes a discovery that explains sperm and eggs are also cells.
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German botanist Carl Heinrich Braun amends the cell theory by calling cells the basic unit of life.
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A German physiologist/physician/pathologist Rudolf Virchow makes an addition to the third part of the cell theory. 3. Cells come only from the reproduction of existing cells. Virchow was also one of the first to prove that diseased cells come from existing healthy cells.
