174 LIFE OF ELIE METCHNIKOFF 



therefore a powerful means of defence. This defence 

 is due to the extreme sensitiveness of the protoplasma 

 of vegetable cells : they react against irritation by a 

 defensive secretion. Vegetables, as well as unicellular 

 beings, can accustom themselves or become artificially 

 accustomed to noxious influences and acquire im- 

 munity. 



As to animals, Metchnikoff had already proved 

 long ago that they defend themselves against morbid 

 agents by phagocytosis, i.e. by intracellular digestion. 

 It is always to be found in cases of immunity and is 

 indispensable to it, on the same grounds as in uni- 

 cellular beings. The organism of multicellular animals 

 possesses various cells which play the part of phago- 

 cytes. There are some in the blood and humors, as 

 also in the divers organs and in the tissues. These 

 phagocytes are either mobile — leucocytes, or fixed — 

 tissue-cells. However, all those cells may be classed 

 into two principal groups : the microphages and the 

 macrophages. Both categories of cells are capable of 

 digesting microbes, but it is chiefly done by the micro- 

 phages, whilst macrophages more especially digest 

 figured elements (cells) of animal origin and poisons. 

 It may be said that the microphages are vegetarians 

 whilst the macrophages are chiefly carnivorous. 



What, then, is the mechanism of phagocytic 

 digestion ? 



Intracellular digestion by phagocytes is accom- 

 plished by means of digestive ferments, similar to 

 those of our own digestive organs. " In both cases," 

 says Metchnikofl, " a diastasic action is due to soluble 

 ferments produced by living elements. In intra- 

 cellular digestion, the diastases digest within the 

 cells, whereas in extracellular digestion the pheno- 



