Which experimental setup helped demonstrate that microorganisms come from the air rather than spontaneous generation?

• A. Petri dish
• B. Swan-necked flask
• C. Beaker with sealed lid
• D. Test tube with cotton plug

Answer: (B)

The idea being tested is that life comes from existing life in the environment, not from nothing inside a closed container. The swan-neck flask lets air get in and out, but its long curved neck traps dust and microbes before they can reach the sterile broth. Dust particles and any germs in the air would have to travel up and over the bend, which is unlikely, so the interior stays free of contamination. If microbes could appear spontaneously inside, the broth would become contaminated even with the curved neck, but it doesn’t, showing that microorganisms come from the air rather than arising on their own. Other setups either block air completely, allow direct entry of dust, or don’t separate the source of contamination, so they don’t demonstrate the air as the source as clearly.