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 Principle that all living things come from other living things.  Before the 17 th 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. 2

 Italian scientist who noticed and described the different developmental forms of flies. › Tiny wormlike maggots turned into sturdy oval cases. › From these cases, the flies eventually emerged. › He also observed that these maggots always seemed to appear where adult flies had previously landed.  These observations caused him to question the idea that flies were spontaneously generated from rotting meat. 3

 Figure 14.1, page 261  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 › Netting-covered jars which contained meat  Control Group › Uncovered jars which also contained meat 4

 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 convincingly that flies come only from eggs laid by other flies.  Redi’s hypothesis was confirmed.  Major strike against the hypothesis of spontaneous generation. 5

 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. 6

 Italian scientist, Lazzaro Spallanzani, designed an experiment to test the hypothesis of spontaneous generation of microorganisms.  He knew that microorganisms grew easily in food.  He tested their growth in meat broth. 7

 Figure 14.2, page 262  Boiled meat broth 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. 8

 Control Group › Flasks were left open  Results › Broth in the experimental group remained clear, free of microorganisms. › Broth in the control group became cloudy due to contamination with microorganisms.  Spallanzani’s Conclusion › Boiled broth only became contaminated when microorganisms in the air entered the flask. 9

 Opponents objected to his method.  They disagreed with his conclusions.  They claimed that he had heated the flasks too long, which destroyed the “vital force” that was in the air inside them.  Without the “vital force” the air could not give rise to the microorganisms.  Because of this rebuttal, spontaneous generation was kept alive for another century. 10

 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.  Figure 14.3, page

 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. 12

 Experimental Group › Curve-necked flasks › Broth remained clear up to a year › When Pasteur broke off the curved necks, the broth became cloudy and 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. 13

 Those who believed in spontaneous generation conceded and gave up their fight.  The principle of biogenesis became a cornerstone of biology. 14