38 VETERINARY BACTERIOLOGY 



and are sometimes assigned to one group, sometimes to another. 

 The yeasts and the molds also show intermediate types. 



Form, Size, and Grouping of Yeasts. Yeast cells are usu- 

 ally spherical, oval, ellipsoid, or cylindrical. For the most part 

 they are larger than bacterial cells, although there are excep- 

 tions. The true yeasts multiply not by fission, but by a process 

 of budding. The cells commonly remain united for a time, giv- 

 ing rise to masses consisting of many individuals. Sooner or 

 later they break apart. The relative shape, size, and groupings 

 of the yeast cells are used in the differentiation of species. In 

 some species part of the cells become considerably elongated and 

 form a kind of false mycelium resembling that of the molds. 

 This character is not always constant in a given species, it may 



Fig. 18. Types of yeast cells and groupings. 



appear when the organism is growing in orte kind of medium and 

 not appear in another. 



Histology and Structure of the Yeast. The very young cell 

 has no cell wall, but by the time it is one-third grown the wall 

 appears as a delicate membrane. In old cells it is sometimes of 

 considerable thickness. Its composition has not been certainly 

 determined, probably it is a carbohydrate or related compound, 

 and not chitinous, as are the walls of bacterial and mold cells. 

 To this substance the name yeast cellulose has been given. It 

 has not been prepared free from nitrogen, so that it is possible 

 that it may be nitrogenous in nature. It is sometimes surrounded 

 by a gelatinous excretion or capsule, as is the case with bacteria. 

 -The yeast cells are never motile. 



Yeast Protoplasm and Cell Inclusions. The contents of the 



