MORPHOLOGY AND ANATOMY OF YEAST CELLS. 293 



which often spreads up the wall of the vessel forming a com- 

 plete ring. Such a complete film-formation only takes place 

 when the primary fermentation is completed. If the flask is 

 shaken, shreds of the skin are loosened and sink, and in this 

 way a complete layer may be collected on the bottom, whilst 

 the skin reforms and assumes a mottled appearance, the 

 younger portions being thin and dark, whilst the older are 

 thick and pale in colour. 



The necessary condition to enable the film to form is the 

 presence of a free and undisturbed surface with access of air. 

 A vigorous film-formation assumes a free access of air. The 

 function of film-formation is subject to the same conditions 

 as the formation of endospores. 



Along with film-formation a bleaching of the wort takes 

 place, which now assumes a light yellow colour. This occurs 

 more rapidly at a high temperature, and is most readily 

 observed in those species which bring about the most vigorous 

 film-formation. Erlenmeyer flasks half-filled with wort and 

 oovered with filter paper are admirably adapted for such 

 cultures. A few drops of a young and vigorous growth of 

 yeast should be introduced. 



Hansen undertook the following determinations : 



(1) The temperature limit for the formation of films. 



(2) The approximate time required for the first appearance 

 of the film at different temperatures. 



(3) The microscopical appearance of the growth at different 

 temperatures. 



The main object of comparative observations of this kind 

 lies in determining the microscopical appearance of films at 

 similar temperatures. 



The examination of the film was undertaken when it had 

 just developed sufficiently to be visible to the naked eye. 



A glance at the illustrations representing these film-growths 

 (see description of species) will show that their general character 

 differs from that of the sedimentary forms. For instance, the 

 sedimentary form of S. cerevisice I. is oval or spherical, whilst 

 in the film, elongated and mycelial cells quickly appear, and 

 the growth gradually assumes an appearance quite distinct 

 from that of sedimentary yeast. 



