YEASTS. 29 



favorable conditions, of developing into an ascus. Many unsuccessful 

 attempts have been made to connect the true yeasts genetically with 

 various forms of fungi such as Mucor, Ustilago and Dematium. At 

 present they must be considered as distinct species. 



GENERAL MORPHOLOGICAL CHARACTERISTICS OF THE TRUE YEASTS. 

 The yeast cell consists of an enclosing membrane and its contents. It 

 may be normally nearly spherical (S. cerevisice), ellipsoidal (S. ellipsoi- 

 deus) or sausage shaped (S. pasteur ianus) . 



The cell wall is thin at first, becoming thicker with age (0.5^ to i.o/i) 

 It consists of two or more layers and may be shown distinctly by treat- 



FIG. 12. Yeast cell. (Original.) 



ment with glycerin, salt solution or dilute acids or alkalis. Under 

 certain conditions, it may give off a mucilage which forms a gelatinous 

 network, enclosing the cells and forming a kind of zooglea. This occurs 

 most noticeably in film or ring formation where oxygen has free access. 

 The cell contents consist of a network of protoplasm, containing a 

 single nucleus near one end. This nucleus can be seen only after harden- 

 ing and staining. In young cells the contents appear transparent and 

 homogeneous. As the cell enlarges, one or more vacuoles or cavities 

 filled with cell-sap appear. In each vacuole are usually one to three 

 highly refringent bodies, the vacuole- granules, which appear to be de- 

 composition products. They are extremely minute, show brownian 

 movement, and stain rapidly with a very dilute solution of neutral red. 

 In old cells, the contents become less transparent and numerous refrin- 

 gent granules appear. These granules are easily stained with methyl- 



