CHAPTER XLV 



THE YEASTS 

 {Blastomycetes, Saccharomycetes) 



The yeasts or blastomycetes form a distinctive family among uni- 

 cellular microorganisms, characterized essentially by their method of 

 multiplication by budding. By this, they are sharply separated from 

 the bacteria. Their differentiation from the higher fungi, the hypho- 

 mycetes, however, is less definitely established, since the chief character- 

 istic of this latter class, the formation of hyphse and mycelial threads, has 

 occasionally been described for some of the forms otherwise identified 

 with the yeasts. It is probable that a gradual transition between the 

 two families exists, represented by a number of connecting forms, some- 

 times spoken of as oidia. For the practical purposes of the bacteriolo- 

 gist, the yeast famUy is sufficiently distinct, both morphologically and 

 biologically, to make a separate classification extremely useful. 



The yeast cell, as a rule, is oval, but among the wild yeasts, or 

 "torulse," spherical forms are common. In size, great variations occur, 

 but in general the yeasts are much larger than bacteria, measuring usually 

 from 10 to 20 micra in length with a width of about one-half or two-thirds 

 of the long diameter. They possess a well-defined, doubly-contoured 

 cell-membrane, composed chiefly of cellulose, and their body protoplasm, 

 unlike that of the bacteria, shows definite structure. Within a mass 

 of finely granul arcytoplasm, a number of highly refractive globules and 

 vacuoles may be observed. Some of the globules have been interpreted 

 as fat-droplets. Other granules, revealed by special staining methods, 

 are interpreted as nuclear material. 



When budding takes place, the mother cell sends out a small, 

 globular evagination of the cell membrane into which maternal proto- 

 plasm flows. This bud gradually enlarges until it has attained approxi- 

 mately the same size as the original cell. Until that time, free inter- 

 communication between the protoplasm of mother and daughter cell 

 exists. Finally, by gradual narrowing of the isthmus connecting the 

 two, the daughter cell becomes complete and free. By some observers 



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