LIFE OF ELIE METCHNIKOFF 175 



menon takes place outside the cells, in the cavity of 

 the gastro-intestinal tube." 



Only gradually has intracellular digestion given 

 place to the digestion by secreted juices. The link 

 between these two modes is to be found in certain 

 transparent Invertebrates, such as the floating mol- 

 lusc Phyllirhoe. The nourishment is first digested 

 in the cavity of the digestive tube by secreted juices, 

 and its treatment is completed within the amoeboid 

 cells of the caecum. 



In higher animals, the digestion of food is due 

 to several digestive ferments (rennet, pepsin, tryp- 

 sin, enterokinase, etc.) produced by divers organs 

 (stomach, pancreas, intestines). The phagocytes 

 also manufacture several digestive ferments; their 

 principal digestive juice is a soluble ferment of the 

 trypsin category, to which Metchnikoff gave the 

 name of cytase. 1 



To the morphological difference of the phagocytes 

 corresponds also a difference in the properties of their 

 cytases, which are suited to the digestion of this or 

 that food. The cytases are kept within the interior 

 of the cells and only escape into the humors when 

 the phagocytes are damaged (Pfeiffer's phenomenon). 

 This kind of ferment does not withstand a tempera- 

 ture above 55 to 58 C. In natural immunity, it 

 plays the principal part by digesting morbid agents 

 inside the phagocytes like any other food. But, in 

 artificial immunity., other soluble ferments come into 

 play, developed in consequence of vaccination. 



The principal of those is the fixator. 2 It is less 



1 It ia also called alexine or complement by other writers. 



2 Designated by other writers by various synonyms : preventive, or 

 sensibilising substance, immunising body, amboceptor. 



