<P> In 1837, Charles Cagniard de la Tour, a physicist, and Theodor Schwann, one of the founders of cell theory, published their independent discovery of yeast in alcoholic fermentation . They used the microscope to examine foam left over from the process of brewing beer . Where Leeuwenhoek described "small spheroid globules", they observed yeast cells undergo cell division . Fermentation would not occur when sterile air or pure oxygen was introduced if yeast were not present . This suggested that airborne microorganisms, not spontaneous generation, was responsible . </P> <P> However, although the idea of spontaneous generation had been in decline for nearly a century, its supporters did not abandon it all at once . As James Rennie wrote: </P> <P>... inability to trace the origin of minute plants and insects led to the doctrine of what is called spontaneous or equivocal generation, of which the fancies above - mentioned are some' of the prominent branches . The experiments of Redi on the hatching of insects from eggs, which were published at Florence in 1668, first brought discredit upon this doctrine, though it had always a few eminent disciples . At present it is maintained by a considerable number - of distinguished naturalists, such as Blumenbach, Cuvier, Bory de St. Vincent, R. Brown, &c . "The notion or spontaneous generation," says Bory, "is at first revolting to a rational mind, but it is, notwithstanding, demonstrable by the microscope . The fact is averred: Willer has seen it, I have seen it, and twenty other observers have seen it: the pandorinia exhibit it every instant . "These pandorinia he elsewhere describes as probably nothing more than" animated scions of Zoocarpae ". It would be unprofitable to go into any lengthened discussion upon this mysterious subject; and we have great doubts whether the ocular demonstration by the microscope would succeed except in the hands of a disciple of the school . Even with naturalists, whose business it is to deal with facts, the reason is often wonderfully influenced by the imagination...</P> <P> Louis Pasteur's 1859 experiment is widely seen as having settled the question of spontaneous generation . He boiled a meat broth in a flask that had a long neck that curved downward, like that of a goose or swan . The idea was that the bend in the neck prevented falling particles from reaching the broth, while still allowing the free flow of air . The flask remained free of growth for an extended period . When the flask was turned so that particles could fall down the bends, the broth quickly became clouded . However, minority objections were persistent and not always unreasonable, given that the experimental difficulties were far more challenging than the popular accounts suggest . The investigations of John Tyndall, a correspondent of Pasteur and a great admirer of Pasteur's work, were decisive in disproving spontaneous generation with dealing with lingering issues . Still, even Tyndall encountered difficulties in dealing with the effects of microbial spores, which were not well understood in his day . Like Pasteur, he boiled his cultures to sterilize them, and some types of bacterial spores can survive boiling . The autoclave, which eventually came into universal application in medical practice and microbiology to sterilise equipment, was not an instrument that had come into use at the time of Tyndall's experiments, let alone those of Pasteur . </P>

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