126 BACTERIA. 



protoplasm, which can be filtered, calcined, and weighed ; 

 after the process of self-digestion, which goes on when the 

 yeasts are deprived of saccharine fluids, a further quantity of 

 9 per cent, is transformed from insoluble protoplasm into 

 the soluble elements above mentioned. The protoplasm 

 has, in fact, been living on itself, and, showing how indis- 

 solubly these metabolic changes are associated with the 

 process of fermentation, both alcohol and carbonic acid gas 

 are formed in the process. Yeast, then, has the power of 

 disassimilating its substance by a series of steps into simpler 

 bodies, but under favourable conditions it may be said to 

 exert its metabolic power only in breaking down in a very 

 superficial way large quantities of the medium on which it is 

 grown — sugar. It is specifically adapted to break down this 

 sugar as far as the stage of alcohol formation, but most yeasts 

 cannot carry disintegration further. Other organisms, how- 

 ever, have the power of carrying on the process as far as the 

 formation of acetic acid for example, at which point another 

 organism may again intervene and take up the work. We 

 have thus the yeast forming alcohol and then dying out as 

 it were, then the Mycoderma aceti comes in and does its 

 work, and later various putrefactive organisms may continue 

 the breaking-down process. On the other hand it must be 

 remembered that whilst yeast sets up the alcoholic fermenta- 

 tion, Hueppe's lactic acid bacillus gives rise to the production 

 from sugar of lactic acid, whilst the Bacillus amylobacter or 

 the bacillus of the butyric fermentation causes the sugar 

 to be split up directly into butyric acid. We have here 

 an example of three specific actions or fermentations of the 

 same substance by the intervention of three different sets of 

 organisms, and it is quite possible that other organisms effect 

 an even further transformation into H^O and COj at one 

 step, the organism being specially adapted at each stage for 

 the work that it has to carry on, or perhaps it would be 

 better to say that the conditions are adapted to the organism. 

 As might be expected, such self-digesting yeast is materially 

 weakened ; it is no longer in a position to absorb much 

 oxygen, and, if the process be long enough prolonged, both 

 the power of absorbing oxygen and the power of inducing 

 fermentation are lost. But if the water that has been used 

 to wash yeast, t'.e., water containing the soluble products 

 necessary for the perfect nutrition of the yeast cells, be 



