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Principle that all living things come from
other living things.
 Before the 17th century (1600s), another
idea was widely accepted.
 Spontaneous generation

› The idea that living things could arise (come
from) nonliving things.
 i.e., maggots appeared on rotting meat;
 fish appeared in ponds that had been dry the
previous season – people thought the mud
gave rise or life to the fish.
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Italian scientist
 noticed, described different
developmental forms of flies.

› Wormlike maggots – sturdy oval cases.
› Flies emerged from cases.
› Observed that maggots always appeared
where adult flies had previously landed.

Caused him to question spontaneous
generation from rotting meat.
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
Redi’s experiment conducted in 1668 to
test his hypothesis.
› Meat kept away from adult flies would
remain free of maggots.
He performed a controlled experiment.
 Experimental Group

› Jars contained meat – covered with nets

Control Group
› Uncovered jars which also contained meat
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
In the experimental group
› The netting allowed air to enter, yet prevented
the adult flies from landing on the meat.

After a few days
› Maggots swarmed over the meat in the
uncovered jars.
› The net covered jars remained free of maggots.
Redi’s experiment showed flies come only
from eggs laid by other flies.
 Redi’s hypothesis was confirmed.
 Major strike against the hypothesis of
spontaneous generation.

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A new tool enters the picture about the
same time that Redi was performing his
experiments.
 Microscope
 New revelation

› World teeming with tiny creatures

Discovered
› microorganisms are simple in structure
› amazingly numerous and widespread

Microorganisms believed to arise from a
“vital force” in the air.
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Italian scientist
 designed an experiment to test
hypothesis of spontaneous generation of
microorganisms.
 Knew that microorganisms grew easily in
food.
 Tested their growth in meat broth.

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Boiled meat broth
 This was to kill all microorganisms already
present in the broth, on the glass of the
flask and in the air within the flask.
 Experimental Group

› Boiled clear, fresh broth in a straight neck
flask until flask filled with steam.
› Sealed the flasks by melting their glass necks
closed while the broth was hot.
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
Control Group
› Flasks were left open

Results
› Broth in the experimental group remained
clear, free of microorganisms.
› Broth in the control group became cloudy
› Contamination with microorganisms

Spallanzani’s Conclusion
› Boiled broth contaminated when
microorganisms in the air entered the flask.
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Opponents objected to his method.
 They disagreed with his conclusions.
 Said he had heated the flasks too long
 This destroyed the “vital force” in the air
inside the flasks.
 Because of this, no microorganisms
would form in the broth.
 Because of opposition, spontaneous
generation was kept alive for another
century.

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 Spontaneous
generation
controversy grew intense by the
mid-1800s.
 Paris Academy of France
› Offered prize to anyone who
could solve this dilemma.
› The winner was Louis Pasteur.
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Pasteur expanded on Spallanzani’s
experiment.
 Instead of using a straight-neck flask, he
made a curved-neck flask.

› This allowed air inside the flask to mix with the
air outside the flask.

The curve in the neck of the flask
› prevented solid particles, such as
microorganisms, from entering the body of
the flask.
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
Experimental Group
› Curved-neck flasks
› Broth remained clear up to a year
Pasteur broke off the curved necks
 Broth became cloudy & contaminated
with microorganisms in a day.
 Pasteur’s Conclusion

› In comparing his experimental group to
Spallanzani’s control group, Pasteur
reasoned that the contamination was
caused by microorganisms in the air.
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
Those who believed in spontaneous
generation conceded and gave up their
fight.

The principle of biogenesis became a
cornerstone of biology.
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