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The Cell Theory evaluates cells as the basic build block of life. It outlines certain principles of how cells and cellular existence works. http://biology.about.com/od/biologydictionary/g/celltheory.htm That link should give a brief view of the principles within the Cell Theory. Many different scientist contributed to the formation of this theory via different experiments performed from around the 17-19th centuries. Another excellent link to a timeline of this theory's creation is below: http://www.timetoast.com/timelines/development-of-cell-theory Before the Cell Theory, it was a commonly held belief that some life formed spontaneously. In order to explain the concept of the spontaneous generation, here are some examples. While we know that it is the moisture that causes worms to show up more often on the surface of soil after a rain, spontaneous generation might have suggested that the rain simply mixed with the soil and created the worms. Another common example was that it was thought for a long time that rotting meat simply created maggots. One scientist, John Needham, did an experiment to evaluate the validity of spontaneous generation. In it, he boiled broth, allowed the broth to cool, and then sealed the containers. His thought was that the boiling would sterilize the broth, so if any microorganisms grew in the broth, they must have generated spontaneously. His broth grew microbes, which allowed the idea of spontaneous generation to survive. (We now know today that he did not use proper sterile technique, nor boil the water long enough, and would all the broth to cool before sealing the containers which allowed contamination.) While there were other scientist who challenged spontaneous generation, Louis Pasteur is most credited with its abolishment. He set up an experiment similar to Needham's using boiled broth. However, he boiled the broth in a flask with a neck that curved downward. This downward curve would not allow for any contaminants to get into the broth. This curve showed that broth, once sterilized by boiling, would not spoil. However, if he turned the flask to allow contaminants into the flask, the organisms would grow again. With this experiment, he discredited the experiment of Needham's and the belief that life sometimes generates spontaneously. From this experiment came a principle that cells must arise from pre-existing cells. Hope this long winded story helps!