MORPHOLOGY AND CLASSIFICATION I47 



from four to eight typical ascospores. Yeasts are very important 

 in the fermentation industries. Very few of them are pathogenic. 

 Among themselves, the yeasts are subdivided into two groups, 

 (i) those which produce ascospores {saccharomycetes or true 

 yeasts) and (2) those which fail to produce such spores (torula 

 or wild yeasts). They are further distinguished by differences 

 in the form of the cells, but especially by differences in physio- 

 logical characters, such as the fermentation of sugars and the 

 production of pigments. 



In the yeasts there is no definite differentiation of cells. Vari- 

 ous cell structures such as cell-wall, nucleus and -cytoplasm with 

 vacuoles and granules, can be demonstrated. The cell membrane 

 is, as a rule, more delicate than in the molds. It sometimes 

 secretes a gelatinous material which forms a thick capsule about 

 the cell. The nucleus is shown by appropriate methods of stain- 

 ing as a single more or less sharply defined mass of chromatin. 

 Under suitable conditions the true yeasts produce endospores, 

 usually multiple, and as many as eight in one cell. These are 

 spherical or ovoid masses surrounded by a definite wall, and usually 

 about haK the diameter of the yeast cell. When supplied with 

 nutriment these spores swell and burst the mother cell, and then 

 begin at once to multiply by budding. Dry commercial yeast 

 cakes contain spores of yeast along with bacteria and molds; moist, 

 "compressed," yeast contains vegetating yeast cells, also mixed 

 with other organisms. 



Bacteria. — Bacteria {schizomycetes) are minute unicellular 

 organisms 0.2 to 4/* in width, which multiply solely by simple trans- 

 verse division (fission), ordinarily resulting in the production 

 of two cells of equal size. In many instances the cells remain 

 attached to each other so as to form long filaments. 



Trichobacteria.^ — Certain of them grow into long filaments 

 without dividing at once into shorter segments. These forms 

 which are classed as higher bacteria or trichobacteria, suggest 

 a very close relationship to the molds and may, perhaps, be re- 

 garded as intermediate between the molds and the lower bacteria. 



