Answer
The significant difference between the flasks used by Pasteur and Spallanzani is that Pasteur used Swan neck flasks, which allowed the entry of air and no other thing, while on the other hand Spallanzani melted the necks of flasks, hence blocking everything. Pasteur allowed dust to settle in some of the flasks and broke the neck of some of the flasks. Microbes clouded the water of the broken flasks while there were no microbes in the flask allowing just air. Hence spontaneous growth theory was nullified.
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