128 BACTERIA. 



(This may also be done by using the Pasteur-Chamberland 

 filter, which keeps back the yeast-cells, but allows the enzyme 

 to pass.) It is only on the addition of living yeast, or that 

 yeast in which there is active protoplasm capable of actually 

 digesting and transforming food materials, that the levulose 

 becomes transformed into dextrose. 



We have here three very significant statements, all 

 of them attested by eminent authorities, which seem to 

 explain many of the anomalies and disagreements in indi- 

 vidual statements. The process of inversion is reduced to a 

 purely chemical one, in which the chemical reagents bring- 

 ing it about are manufactured by the living protoplasm ; 

 apart from which, however, it is able to exist and act so that 

 saccharose is converted into dextrose and levulose merely 

 by the action upon it of the chemical products or enzymes 

 contained in yeast water ; whilst the conversion, and cer- 

 tainly the fermentation of levulose, can only take place when 

 this material comes into actual contact with the living pro- 

 toplasm, from which it seems, necessarily, to be more closely 

 associated with the actual process of digestion. 



This is a decided step in advance in assisting to explain 

 the process of fermentation, of digestion in the higher 

 animals, and of those changes that take place in putrefactive 

 and pathogenic processes. It also appears to throw light on 

 some of the points in dispute on fermentation. 



For example, we can easily understand how different the 

 results would be in cases where experimenters use yeast as 

 the fermenting agent, and crude saccharose as the fermentable 

 substance in one case, and yeast with dextrose in another. 



The process of inversion of the saccharose by the invertin 

 into dextrose and levulose is carried out easily enough, 

 whether oxygen be present or not, and this dextrose is easily 

 broken up by the yeast. When we come to the levulose, 

 however, it is a different matter, as this substance can 

 apparently only be converted into dextrose when it is in 

 direct contact with the protoplasm of the cell, and so long as 

 food materials and oxygen can be obtained from other sources 

 there is not the same tendency for the protoplasm to take up 

 this levulose that there is when other sources of food supply 

 are cut off. The active and rejuvenated cells are still able to 

 utilize the levulose, whereas older cells, which, as we have 

 seen, after self-digestion lose even their power of transforming 



