238 Proceedings. 



but after this the action becomes weakened and the ferments 

 cease to hve if not again brought in contact with free 

 oxygen. (3) The number of generations in which the 

 ferment-organisms can exist without free oxygen has not 

 yet been definitely determined ; but it seems to be greatest 

 with SaccJiaroniyces cerevisi(2. Yeast follows the general 

 laws of digestion, for it not only assimilates bodies from the 

 surrounding liquid, which it uses for its nourishment, but 

 it also excretes substances into it which are of no further 

 use. After a cell has reproduced several times, its time of 

 life expires, but the cell does not immediately become 

 inactive, for the membrane allows fluids to pass in and out 

 of it until equilibrium is established between it and the 

 outer liquid. These statements explain the manner of 

 nourishing and multiplication of yeast, but do not explain 

 the cause thereof It has been argued: — (i) The yeast 

 cell consumes the nutritive parts of the fermentable liquid, 

 and excretes alcohol, carbonic acid, and other products. 

 This theory assumes fermentation to be a purely physio- 

 logical act ; a small portion of sugar is used for the 

 construction of new cells, but the greater portion is thrown 

 off in a form useless to the ferment-organisms. According 

 to this idea the production of new yeast must be propor- 

 tionate to the amount of fermentation products. (2) The 

 yeast consumes only as much of the nutritive parts of the 

 fermentation liquids as it needs for its nourishment and 

 reproduction ; in its excretiaments one or more combinations 

 are formed which have the power of converting sugar into 

 fermentation products. This theory is purely a chemical 

 one. The organisms, and the reproductions thereof, have 

 nothing to do with the fermentation. The function of the 

 yeast is to produce that body or bodies which act as fer- 

 ments. If these real ferments could be artificially produced 

 without the intervention of organisms the theory would be 

 fully established. (3) The yeast cell nourishes itself on 



