630 PATHOGENIC MICROORGANISMS 



definite karyokinetic changes in the nuclear structures have been de- 

 scribed as accompanying the budding process. This observation, how- 

 ever, has not been generally confirmed. Under conditions of special 

 stress, such as unfavorable environment or lack of nutrition, most 

 yeasts possess the power of forming spores. These, called " ascospores," 

 are formed within the yeast cell itself, each spore forming a separate 

 membrane of its own, but all of them lying well protected within the 

 original cell-membrane. The number of ascospores formed is constant 

 for each species, and rarely exceeds four. 



The yeasts have been studied most extensively in connection with 

 fermentation and are industrially of great importance in the production 



FIG. 144. YEAST CELLS. Young culture unstained. (After Zettnow.) 



of wine and beer. Although Schwann, as early as 1837, recognized the 

 fact that many fermentations could not occur without the presence of 

 yeast, it was not until considerably later that the study of yeast fermen- 

 tation was put upon a scientific basis. The typical fermentative action 

 consists in the transformation of sugar into ethyl alcohol according to 

 the following formula: 



C 6 H 12 6 = 2 C 2 H 5 OH + 2 C0 2 



The enzyme by which this fermentation is produced is known as "zy- 

 mase," and is, according to Buchner, in most cases, an endo-enzyme 

 which may be procured from the cell by expression in a hydraulic 

 press. In addition to this, however, the various yeasts also produce 



