550 TRICHOMYCETES, ACTINOMYCETES, HYPHOMYCETES 



refractile bodies ascopores within their cytoplasm when inviron- 

 mental conditions become unfavorable for further development and, 

 unlike the bacteria, each yeast commonly produces more than one 

 spore, usually two, three or four, but rarely or never more than four. 

 The ascospore is outlined by a doubly contoured membrane and 

 usually it remains within the intact maternal cell. At sporulation 

 each ascospore develops into a mature yeast cell, consequently 

 sporulation in this group is, in a sense, a process of reproduction, for 

 each ascospore is potentially equivalent to a bud in that it develops 

 into a complete vegetative cell. 



The yeasts are of considerable importance commercially; some 

 varieties are extensively used in the fermentation of malt and others 



FIG. 88. Yeast cells showing budding. 



are employed in the manufacture of bread. In either case the organism 

 liberates carbon dioxide from carbohydrates, and alcohol as well. 

 This activity is brought about by an intracellular enzyme, "zymase," 

 which may be obtained in an active state, free from yeast cells, by 

 crushing the latter with hydraulic presses and filtering off residual 

 cells through porcelain filters. Little or no acid is formed and the 

 yeast fermentations are, in general, different in this respect from 

 bacterial fermentations in which acid formation, but not alcohol 

 formation, is the rule. 



Structurally, yeasts exhibit greater complexity than the bacteria. 

 The cytoplasm of the yeast cell usually exhibits a granular or vacuo- 

 lated appearance and nuclear material, or at least structures that 

 color like nuclei have been demonstrated. 



The view was formerly held that yeasts had some etiological rela- 



