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Redi's experiment
Francesco Redi's experiments were designed to refute the idea of spontaneous generationt. Redi left the jars of meat, some open, some covered with gauze, and some sealed. Worms only appear in open jars, demonstrating that organisms arise from other organisms and they can't just arise from non-living materials. -
Needham's rebuttal
John Needham conducted experiments that seemed to support the idea of spontaneous generation. Needham boiled broth and sealed it in containers. Later, he found microorganisms in the broth and concluded that life could spontaneously arise. Spallanzani's experiments, which used better controls, cast doubt on Needham's findings and supported the idea that life only comes from pre-existing life. -
Criticism from Spallanzani
Lazzaro Spallanzani criticized John Needham's experiments, because he realized that Needham's sealing method was the problem. Spallanzani believed that the container used by Needham was not correctly sealed, this allowed contaminants to enter and microorganisms to grow in the broth. Spallanzani's experiment showed that when the containers are properly sealed, no microorganisms emerge. This contradicted the theory of spontaneous generation again. -
Pasteur puts spontaneous generation to rest
Louis Pasteur re-created the broth experiments but this time with a swan-neck flask. This flask allowed air to enter easily but all the dust and the microorganisms were stucked in the curved part of the neck. That's why there was no life in the broth for one year, until he broke off the top of the bottle. A few days later he noted life forms in the broth. This experiment demonstrated that microorganisms do not appear in a sterile environment unless it's exposed to contaminants from the air.