But when more knowledge was gathered about living organisms, it became evident that this doctrine of Spontaneous Generation was not true. Earlier it was thought that maggots are formed in meat spontaneously. But Italian physician, Francesco Redi showed that the maggots are nothing but larval stages of the flies & will never appear if the meat is protected by placing it in vessel covered with fine gauze. In that case flies are not able to lay eggs on the meat. This experiment is of utmost importance as it gave a much needed proof against Spontaneous Generation theory. For technical reason it was very difficult to prove that microorganisms are growing inside the organic infusion not spontaneously but from their earlier life form already present in air or in the organic infusion. It was in the middle of nineteenth century that the cumulative evidence became sufficient to rule out this theory.
In the middle of 18th century Italian naturalist Lazzaro Spallanzani proved that heating can prevent the appearance of “animalcules” in infusions, although the duration of heating varies. He concluded that air is the carrier of animalcules in well heated infusions, he closed the flasks containing infusion hermetically (earlier others closed the vessel by mechanical plugging which is not sufficient to prevent the entry of air completely) and boiled the infusion. The boiled infusions are free of animalcules but if there is a tiny crack in the bottle the animalcules reappear. This strongly suggests air is the carrier of animalcules, & they are not generated spontaneously.