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Jan Baptist van Helmont discovered spontaneous generation. He said that the supposed production of living orgnaisms comes form nonliving matter.
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Aristotle theorized how plants and animals developed. The actual date was in 384 B.C. but I wanted the timeline to scale out correctly.
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Jansen invented the first basic microscope. Without this invention, people would have never seen cells
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Robert Hook discovered the first cells and gave them their name. Without him, cells wouldn't have been discovered until later in life.
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Leeuwenhoek was the first to witness a live cell under a microscope. He saw bacteria about 9 years later.
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Oken said that all living organisms originate from and consist of cells or vesicles. This might have been the first discovery about Cell Theory.
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Brown discovered what the nucleus does for the cell. Without it, we wouldn't know how the nulceus would help the cell.
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Schwann and Schleiden discovered that some organisms are multicellular and some are unicellular. They also found out that the cell membrane and the cell nucleus belong to the properties of the cell, by comparisons of plant and animal cells. Without them, cell theory wouldn't have been discovered.
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Roelliker realized that sperm cells and egg cells are also under the catergory of cells.
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Virchow said that only cells can develop from existing cells. Without this, the way cells multiply would be unknown.
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Pasteur demonstrated that the microorganisms would grow in sterilized broth only if the broth was first exposed to air that contained their spores (reproductive cells).
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sceince.howstuffworks.com/dictionary/famous-scientists/chemists/louis-pasteur-info.htm www.xtimeline.com/evt.view.aspx?id=940911 http://science.howstuffworks.com/innovation/scientific-experiments/scientific-method4.htm physicsbook.gr/index.php?option=com_content&view=article&id=126%3Aschwann-and-schleiden&catid=64%3Abiology&Itemid=92 bitesizebio.com/articles/history-of-cell-biology/