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Redi's theory was based on the fact that at some point some adult insect had deposited its larvae on decomposing matter.
To demonstrate this, he put raw meat in three different containers, one of them open and the other two with gauze, thus allowing the passage of air but not of larger insects. In the result, larvae could be seen in the first container, while in the other two, only the eggs were found in the gauze, not inside. -
This Italian naturalist demonstrated that the organisms come from other already existing organisms based on an experiment that consisted of putting meat broth in two sealed containers, at a high enough temperature to kill all existing microorganisms and thus verify that no larva emerged from there. -
Finally, Pasteur just demonstrated this theory, introducing meat broth into two long-mouthed distillation bottles, thus allowing entry into the air but leaving the microorganisms at the base, without directly touching the meat.
He then proceeded to sterilize the containers and wait a few days, without anything emerging, and to finish making sure, he finally cut the mouth of the containers, giving rise, after a while, new microorganisms.