GRADUAL DEVELOPMENT OF THE GERM THEORY. 77 



ledge; but in such a complicated question, and one 

 requiring the whole efforts of more than one investigator, 

 a definite conclusion was not at first possible, and too 

 rigid a deduction would only have impeded the develop- 

 ment of knowledge. 



In 1857 Pasteur demonstrated that fermentation was Pasteur's 

 most intimately bound up with the life and the growth T1 

 of the yeast cells, and was thus a result of the action of 

 these cells. The growth of the yeast takes place at the 

 expense of the constituents of the fermentescible fluid, 

 and therefore all the sugar cannot be broken up into 

 alcohol and carbonic acid ; on the contrary a portion 

 (about 5 per cent.) is employed for building up the cell 

 constituents and for the formation of bye-products. 

 The fermentescible materials form the nutriment of the 1 

 yeast, and these cells employ a portion for the formation 

 of new cell substance, while the other and much larger 

 portion is transformed in the yeast cell into alcohol and 

 carbonic acid. As the yeast cells consist also of nitro- 

 genous material and mineral substances Pasteur assumed 

 that traces of both these materials must be present in 

 the fermentescible fluids if the yeast was to develop and 

 break up the sugar. Pasteur found at a later period 

 that 3'east could develop in pure sugar solutions free 

 from nitrogenous materials, and could there excite fer- 

 mentation ; but the further development in this case 

 takes place at the expense of a reserve stock of nitro- 

 genous material which fresh yeast cells usually contain; 

 In like manner old dead yeast cells seem to be able to 

 furnish new nutritive material for young cells ; and 

 under certain circumstances, namely, when yeast is 

 mixed with fluid free from sugar, the non-nitrogenous 

 substance (cellulose ?) of the old yeast cells can take the 

 place of the sugar, produce alcohol and carbonic acid, 

 and so give rise to a fermentation of the yeast itself. 



In the year 1860 Pasteur showed that it was not 

 essential that the nitrogenous nutriment of the yeast 

 should consist of albuminous materials, but that salts 

 of ammonia could take their place. Such salts along 

 with mineral substances (which are most easily added in 



