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 



FiO. 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 

 appropriate treatment the latter may 

 be shown to consist of cellulose ; and 

 it is distinctly thicker in old or resting F spores) Four spores inacell 



Cells than in young Ones Or those Vlg- of brewer's yeast (Saccliaromyces 



orously growing. Within the granular cerwfete) - 

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

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



-Spor 



Yeast (As- 



