l66 CELL NUTRITION 



enzyme. It is natural, therefore, that the cell should make use of 

 soluble ferments in preparing the molecule which it has seized. 



We now (thanks to the researches of Fischer, Hierf elder, and 

 others) know that the action of these enzymes is strictly specific, 

 and dependent on a stereo-chemical relation between the ferment 

 and the body acted on, and that it is necessary in all cases for a 

 union to be brought about between the two substances before the 

 specific action is effected. Thus, if the two could unite in the blood, 

 the molecules would be broken down before they reach the cells, and 

 the food material would reach it in an indifferent form, which, 

 however well adapted to some cells, might be useless to others. 

 The substances, therefore, do not unite in the blood-stream at all, 

 but the cell first seizes a molecule of food-stuff, and then one or 

 more ferments which can change it into the exact form which the 

 cell requires. Thus, the process of cell nutrition, according to 

 Ehrlich, consists in the establishment of a link between the food 

 and a suitable ferment after the former has been selected by the 

 cell. 



We may inquire why the cell does not form its own ferments. 

 In certain cases it may do so ; the existence of the endocom- 

 plements lends support to this view. But it is readily conceivable 

 that it may be more economical in every way for the animal 

 to limit this process of enzyme formation to certain specialized 

 cells or tissues, just as pepsin is formed by the cells of the gastric 

 glands. This is probably the case, and all experience goes to 

 show that the cells to which this duty is delegated are the 

 leucocytes. 



Ehrlich, therefore, conceives of the cytotoxic mechanism of 

 amboceptors and complements as being adapted, in the first place, 

 to cell nutrition, and, secondarily, to the defence of the body by 

 bringing about solution of foreign cells. Metchnikoff 's views may 

 be summarized as follows : He holds that both the substances 

 taking part in cytolysis are ferments, and that both are adapted 

 for intracellular digestion. His views on phagocytosis will be 

 discussed subsequently, and here it will be sufficient to give the 

 merest outline. He has shown that in the lowest Metazoa e.g., 

 in the Ccelenterata digestion is an intracellular process entirely ; 

 the hypoblastic cells lining the alimentary canal seize food particles 

 from the lumen and digest them, but form no secretion like those 

 of the higher animals. In the cells which have seized and are 

 digesting food particles in this way he was able to demonstrate 



