BACTERIOLYSIS AND ALLIED PHENOMENA 165 



absorption of more complement. It may be added, however, that 

 many of the recent researches on antibodies point to them as 

 being governed by the very complex laws, as yet not fully under- 

 stood, which regulate the actions of colloids on one another 

 and on crystalloids. This is discussed, though very briefly, in a 

 subsequent chapter. 



As regards the evolutionary significance of the facts of cytolysis 

 (using the word to cover the solution of bacterial and all other 

 cells), there are two theories which, though apparently widely 

 different, have, on ultimate analysis, much in common. We have 

 seen how Ehrlich explains the process of cell nutrition, the role 

 which he attributes to complement, and the method in which he 

 conceives the complex antibodies to be formed. On his theories, 

 therefore, the fundamental process is one of cell nutrition, and 

 we may imagine it to have become adapted to serve as a means 

 of immunity during the course of natural selection : the animals 

 which could make use of their mechanism of cell nutrition as a 

 means of defence against invading micro-organisms would survive 

 and perpetuate their species ; the others could not. 



We may add a few more considerations on this process of 

 cell nutrition. We must assume that the food molecules of 

 proteid which circulate in the blood are in an indifferent form, 

 and are equally well adapted for the nourishment of cells of the 

 brain, liver, etc., this method of transmission of nourishment 

 having been found the most convenient and economical. The 

 first requisite for the nutrition of the cell is that one of these 

 molecules shall be brought into close relations with its protoplasm. 

 Ehrlich speaks of this as being due to the selective action of the 

 cell receptors, but we must not be misled by terms or diagrams, 

 however convenient they may be to enable us to form a mental 

 picture of the process. Ehrlich simply means that certain 

 specialized molecules or groups of molecules have a chemical 

 combining affinity for certain food molecules. This union is the 

 first requisite, and in some cases it may be sufficient, and the 

 molecule may be forthwith incorporated with the cell protoplasm. 

 In most cases, however, this is not so, and the cell has to 

 transform an indifferent molecule of proteid material into a form 

 suitable for assimilation into itself. We now know that most, if 

 not all, of the activities of a living cell in metabolism are dis- 

 charged through the agency of enzyme action, and that each of 

 the metabolic actions of a cell appears to depend on a special 



