and introduced the methods of pure culture which permitted a morphological study of yeasts.
After this, the yeasts were subjected to many investigations. In the meantime, the methods of culture were not quickly taken up, and their perfection was rather slow. It was also difficult for the earlier investigators to distinguish between yeasts and other microorganisms which developed at the same temperature. The early work on yeasts conflicted with the erroneous conceptions of the pleomorphists who maintained that the microorganisms could be reduced to a small number of species capable of exhibiting different shapes dependent upon the conditions. About 1871, Bechamp reported that the acetic acid bacteria could change into yeasts. In 1872, Trecul thought that he had obtained the transformation of the spores of Penicillium glaucum into yeasts. In 1875, Robin stated that many of the yeasts (Torula cerevisiae, Mycoderma cerevisiae) are, with Penicillium glaucum, only forms of the same fungus. A little later, however, very careful investigations were reported. Between 1868 and 1870, Rees observed endospores in many species of yeasts, and gave the first accurate description of these organs. This was followed by the work of Engel, Seynes, Brefeld, and de Bary. The last of these, in his "Morphology and Biology of the Fungi," classed the yeasts among the Ascomycetes.
The introduction and perfection of pure cultures permitted the exact morphological studies of the yeasts. This was the work of Hansen, who is the true founder of this study, and whose name marks a second step in the history of the yeasts. Through his careful investigations for a period of 30 years, this mycologist perfected methods which were introduced by Pasteur for culturing and isolating the yeasts. He succeeded in inoculating cultures with a single cell and separating one species from another. By careful studies on the morphological and physiological properties of yeasts, Hansen found the characteristics which allowed the differentiation of one species from another. He has thus been able to characterize a large number of species the majority of which are known. Hansen is responsible for our knowledge of the life cycle and the systematic relationships of the yeasts. In recent years, he has proposed a classification which has been universally accepted.
The third step in the study of yeasts was the discovery, by Buchner, of zymase, which allowed a considerable advance in the study of yeast nutrition and the mechanism of alcoholic fermentation. Thus, as has been said, three names, Pasteur, Hansen and Buchner, remain intimately associated with the study of the yeasts and will constitute the pivot about which our investigations will center.
