STRUCTURE OF YEAST. 



185 



Microscopical examination proves that the milky appearance 

 of liquid yeasts is due chiefly to the presence of myriads of 

 minute egg-shaped suspended bodies, and that pressed yeast is 

 almost wholly a mass of similar forms. These are the cells of 



/ 



yeast ; which is therefore essentially a mass of unicellular organ- 

 isms. For reasons which will soon appear yeast is universally 



FIG. 94. Yeast-cells. Brewer's (bottom) yeast showing structure protoplasm, cell- 

 walls, vacuoles, fat-drops. (Nuclei not shown.) 



regarded as a plant, and the single cell is often spoken of as the 

 yeast -plant. 



Morphology. The particular yeasts which we shall consider are 

 the common cultivated forms of com- 

 merce. The cells of an ordinary cake 

 of pressed yeast are spherical, sphe- 

 roidal, or egg-shaped in form, and con- 

 sist of a mass of protoplasm enclosed 

 within a well-defined cell- wall. By 



V 



appropriate treatment the latter may 

 be shown to consist of cellulose ; and 

 it is distinctly thicker in old or resting FlG - 95 -- s P res of Yeast (As- 



v ' cospores). Four spores in a cell 



Cells than ill VOUng Ones O1* those vig- of brewer's yeast (SaccJiaromyces 



orously growing. Within the granular cerevism '>- 

 protoplasm (cytoplasm) are usually a number of vacuoles (con- 

 taining sap) and minute shining dots (probably fat-droplets), but 



