INTRODUCTION 3 



a sexuality quite comparable to that which is observed in the lower 

 Ascomycetes. The yeasts make up a family of the Ascomycetes 

 known as Saccharomycetes. 



True yeasts never produce endo- or ascospores. Do they represent 

 forms derived from more highly developed fungi and made constant 

 by a long adaptation to this condition? Or, are they true Saccharo- 

 mycetes having lost, by a series of unknown circumstances, their apti- 

 tude of forming spores? We shall see that a definite loss of this 

 characteristic has often been proved among the Saccharomycetes 

 during certain special conditions. Be that as it may, the origin of the 

 yeasts is entirely ignored; the Saccharomycetes are then separated 

 and regarded as yeasts of uncertain origin. In this book, we shall 

 examine extendedly only the true yeasts; first, yeasts with ascospores, 

 or Saccharomycetes; secondly, those cells which exhibit all the 

 characteristics of Saccharomycetes with the exception of ascospore 

 formation and which, from the above, one might call pseudo-yeasts. 

 All of those yeast-like structures of other fungi will be neglected. 

 True yeasts are very abundant in nature; over five hundred are 

 known. The limits of our study must be rather wide. 



History of the Study of Yeasts 



The study of yeasts is intimately associated with that of fermen- 

 tation. The idea that alcoholic fermentations are caused by living 

 organisms originated with Linne. In 1680 Leewenhoek first de- 

 scribed the yeasts. He described them as globular bodies, oval or 

 spherical in shape. In 1799 Fabroni compared yeasts to albu- 

 minoids. About 1825, and for some time after this, Mitscherlick, 

 Cagnard-Latour, Schwann, and Ktitzing demonstrated that beer and 

 wine yeasts were composed of cells which multiplied by budding. 

 In 1839 Schwann observed, for the first time, endospores in yeasts. 

 He proved that they might be freed by a rupturing of the cell wall. 



But the nature of yeasts has been definitely known since the 

 period in which Pasteur commenced his investigations on fermenta- 

 tion. Up to this time, it was known that beer yeast multiplied 

 when introduced into saccharine wort; it was believed that it was 

 formed spontaneously and that, in the yeast, was an occult force 

 which produced the fermentation; that was all there was to it. 

 With Pasteur, definite knowledge with regard to yeasts commenced. 

 It was in 1859 that he established, by his memorable experiments 

 which cannot be reproduced here on account of the lack of space, 

 that fermentation is correlative with the life of yeasts. Some years 

 later, he demonstrated the impossibility of spontaneous generation 



