Gay-Lussac's speculation that oxygen caused fermentation. It suggested that some sort of microorganism was necessary for the process to happen. Next, Schwann tested the effects of purified air and unpurified air. He sterilized the air by passing it through heated glass bulbs. Fermentation did not occur in the presence of purified air. It did occur in the presence of unpurified air, suggesting that something in the air started the process. This was strong evidence against the theory of spontaneous generation, the idea that living organisms could develop out of nonliving matter. Schwann had demonstrated that fermentation required the presence of yeasts to start, and stopped when the yeasts stopped growing. He concluded that sugar was converted to alcohol as part of an organic biological process based on the action of a living substance, the yeast. He demonstrated that fermentation was not an inorganic chemical process like sugar oxidation. Living yeast was necessary for the reaction that would produce more yeast. Although Schwann was correct, his ideas were ahead of most of his peers. They were strongly opposed by Justus von Liebig and Friedrich Wöhler, both of whom saw his emphasis on the importance of a living organism as supporting vitalism. Liebig, in contrast, saw fermentation as a series of purely chemical events, without involving living matter. Ironically, Schwann's work was later seen as being a first step away from vitalism. Schwann was the first of Müller's pupils to work towards a physico-chemical explanation of life. Schwann's view furthered a conceptualization of living things in terms of the biological reactions of organic chemistry, while Liebig sought to reduce biological reactions to purely inorganic chemistry. The value of Schwann's work on fermentation eventually would be recognized by Louis Pasteur, ten years later. Pasteur would begin his fermentation research in 1857 by repeating and confirming Schwann's work, accepting that yeast were alive, and then taking fermentation research further. Pasteur, not Schwann, would challenge Liebig's views in the Liebig–Pasteur dispute. In retrospect, the germ theory of Pasteur, as well as its antiseptic applications by Lister, can be traced to Schwann's influence. Cell theory In 1837, Matthias Jakob Schleiden viewed and stated that new plant cells formed from the nuclei of old plant cells. Dining with Schwann one day, their conversation turned on the nuclei of plant and animal cells. Schwann remembered seeing similar structures in the cells of the notochord (as had been shown