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Redi's experiment
In 1668, Francesco Redi placed meat in spearate jars, one sealed tightly, one covered with a cloth and one open.
The results that Redi obtained were that the opened jar attracted flies, and then maggots appeared. The air-tight jar didn’t attract the flies, there were't any maggots. Finally, the jar covered with a cloth attracted the flies and some maggots appeared outside of the jar.
In conclusion, maggots don't appear from rotten meat, but flies create them, laying their eggs on the meat. -
Needham's rebuttal
In 1745, John Needham, an English clergyman, claimed that spontaneous generation could occur and performed an experiment.
Needham briefly heated broth to its boiling point, to kill microorganisms, and poured it into flasks. Soon after the broth cooled, he sealed them.
Later, he observed living microorganisms in the broth, concluding that spontaneous generation was a fact.
Years later, this experiment was considered incorrect because there were some flaws that altered the result. -
Criticism from Spallanzani
Lazzaro Spallanzani did an experiment in 1765 that proved Needham's conclusions wrong.
Spallanzani sealed flasks filled with broth. Next, he boiled them for a long time. Later, the broth did not have any life. However, once he unsealed the flask, microorganisms rapidly grew in the broth.
Spallanzani concluded that spontaneous generation was false and microbes came from contaminated air.
The differences were that he boiled the broth for longer and the flask was healed during the whole process. -
Pasteur puts spontaneous generation to rest
Louis Pasteur re-created the experiment and left the system open to air. He placed broth in a swan-beck bottle oriented downwards, and observed no life for one year. He then broke off the top of the bottle, exposing it more directly to the air and trapped particles, and after a few days he observed life. He concluded that the contamination came from life forms in the air, not a supposed “life force”. The pollution was more dense, so it remained in the S shaped neck, it didn't get to the broth.