STRUCTURE OF YEAST. 



185 



Microscopical examination proves that the milky appearance 

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

 minute egg-shaped suspended bodies, and tliat 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-ceUs. 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 sj^oken 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-deiined cell-wall. Bv 

 ap2)ropriate treatment the latter may 

 be shown to consist of cellulose ; and 

 it is distinctly thicker in old or resting Fm. n5.-Spore9 of Yeast (As- 



^ 'J ^ ° cospores). Four spores in a cell 



cells than in young ones or those vig- of brewer's yeast (^atc/KiromyftViJ 



orously growing. Within the granular «<^'''*^'''"""')' 

 protoplasm {cyto2)lasm) are usually a number of vacuoles (con- 

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



