Final answer:
Pasteur used a swan-neck flask with a curved neck, while Spallanzani used sealed flasks with a straight neck. Pasteur's investigation settled the dispute about spontaneous generation by demonstrating that bacteria were introduced from the air.
Step-by-step explanation:
The significant difference between the flasks used by Pasteur and Spallanzani was the shape and design. Pasteur used a swan-neck flask with a curved neck that allowed air to enter but prevented the entry of bacterial and fungal spores, while Spallanzani used sealed flasks with a straight neck. Pasteur's investigation settled the dispute about spontaneous generation by demonstrating that sterilized broth remained free of contamination when the flask was intact, but became contaminated when the neck was broken off, proving that bacteria were introduced from the air.