4 INTRODUCTION 



and introduced the methods of pure culture which permitted a mor- 

 phological 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 microor- 

 ganisms which developed at the same temperature. The early work 

 on yeasts conflicted with the erroneous conceptions of the pleomor- 

 phists who maintained that the microorganisms could be reduced to 

 a small number of species capable of exhibiting different shapes de- 

 pendent 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 187, Robin stated that many of 

 the yeasts (Torula cerevisiae, Mycoderma cerevisiae) are, with Peni- 

 cillium glaucum, only forms of the same fungus. A little later, how- 

 ever, 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 isolat- 

 ing 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 Buch- 

 ner, 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 con- 

 stitute the pivot about which our investigations will center. 



