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Your textbook tells you that the idea of spontaneous generation, life arising from "decaying and rotting matter", was finally dismissed by Louis Pasteur (NCERT Class 12 Biology, Chapter 6). Two centuries before Pasteur, an Italian physician tested the same belief with jars of rotting meat. This post covers Francesco Redi's experiments in his own words, the objection he designed against, the mistake he still made, and how to set out his experiment the way an exam expects.
Who was Francesco Redi?
The best source is Redi himself. His book Esperienze intorno alla generazione degl'insetti appeared in 1668, and an English translation by Mab Bigelow, Experiments on the Generation of Insects, was published in Chicago in 1909 from the fifth Italian edition of 1688. The translator's introduction records that Redi was born in Arezzo, Tuscany, in 1626. He served as physician at the court of the Medici in Florence and ran its laboratory, while also writing poetry that is still read in Italy.
The same introduction says he died in Pisa on 1 March 1697. Some reference works, such as the Encyclopedia of Parasitology, give his dates as 1626 to 1698, so you will see both.
What did people believe about maggots?
Redi wrote at a time when the authority of Aristotle on this question was still widely accepted. Aristotle had held that some small animals arise from non-living matter, and the belief was ancient and popular. If meat left out in summer filled with worms, the meat itself seemed to be making them.
Redi noticed something else. He saw that decaying flesh was "covered not alone with worms, but with the eggs from which, as I have said, the worms were hatched", and that flies of the same kind hovered over meat before it became wormy. Butchers already knew to cover meat with white cloths in summer. He began to believe that all worms in meat "were derived directly from the droppings of flies, and not from the putrefaction of the meat". Then he wrote the line that makes him a scientist: "Belief would be vain without the confirmation of experiment".
What exactly was the sealed flask experiment?
In the middle of July, Redi put a snake, some fish, some eels from the river Arno and a slice of veal into four large, wide-mouthed flasks, and sealed them. He set up the same number of flasks in the same way but left them open. The open flasks soon became wormy, with flies "entering and leaving at will". In the closed flasks he "did not see a worm, though many days had passed". Maggots did appear on the paper cover outside, looking for a way in.
The meat in the sealed flasks still rotted. The fish turned into a thick fluid. So rotting happened without maggots, which separated the two things his opponents treated as one. He repeated the test in different seasons and with different vessels, and even buried meat for weeks. It never bred worms unless flies had reached it.
How did he answer the air objection?
A careful critic could say that sealing kept out air, and air might be needed for life to form. Redi saw this coming. He put meat and fish in a large vase "closed only with a fine Naples veil, that allowed the air to enter", and placed the vase inside a frame covered with the same net. No worms appeared in the meat. But flies landed on the outer net and left worms on it.
Worked example: setting out Redi's experiment
Exams often ask you to identify the parts of a controlled experiment. Here is Redi's set-up, written the way an answer should be.
Hypothesis: maggots in meat come from eggs laid by flies, not from the meat.
Independent variable: whether flies can reach the meat (open, sealed, or gauze-covered).
Dependent variable: whether maggots appear in the meat.
Controlled variables: the same kinds of meat and fish, the same season, the same vessels.
Control group: the open flasks, which show that maggots do appear under normal conditions.
Result: maggots only where flies could reach the meat; rotting in all flasks.
Common mistakes:
Calling the sealed flask the control. The open flask is the comparison that shows what normally happens.
Saying Redi disproved spontaneous generation for all life. He tested insects in meat. Microbes were Pasteur's question, in our post on Louis Pasteur and the swan-neck flask.
Forgetting the gauze test. Without it, the air objection stands.
What did Redi still get wrong?
Redi did not reject spontaneous generation everywhere. The translator's introduction explains that, studying insects that come out of plant galls, he expected a fly had laid its egg in the plant. His observations seemed to show the opposite, so he concluded that the plant itself produced the gall insect, giving way to Aristotle's view. His pupil Vallisneri continued the work and corrected the error.
This is worth remembering. The same careful person can test one claim well and accept another too quickly.
What can a student take from his way of working?
Turn a belief into a test. He wrote it himself: belief without experiment is vain.
Change one thing at a time. Open, sealed, gauze: each step answered a specific question.
Expect to be partly wrong. Even good method left him with a mistake about galls.
If you want to practise identifying variables in experiments like this, Fermi's AI tutor can check your understanding as it goes and decide whether to explain, ask or show. For what came next, from Redi's flies to Miller's flask, read Stanley Miller's experiment step by step.
Try it now: design a Redi-style test of the claim that mould on bread comes from spores in the air, and name your control.
