428 SCIENCE PROGRESS. 



wandering cells ; it does not however appear to me on 

 that account permissible for us to consider that the cells 

 themselves are to be classed as similar. 



Study of the varieties of leucocytes brings out clearly 

 the fact that in the wandering cells taken as a group di- 

 gestive activity is prominent, and that the secretion of 

 digestive juices is a function highly developed in them. 

 In some varieties the secretion is poured out into a special 

 "vacuole" in which the food or prey is by the motive 

 power of the cell surrounded ; the digestion takes place 

 then in what may be called an "intracellular" manner. 

 The phenomenon termed "phagocytosis" is a particular 

 instance of this "intracellular" digestion. Metschnikoff, 

 whose attention has been largely devoted to this subject, 

 considers that in "phagocytosis" we have the primary 

 and central fact not only of many normal processes, but 

 also of the great pathological process of inflammation ; 

 he even defines inflammation as a " phagocytic reaction," 

 and attempts to explain all resistance to and immunity from 

 infectious disease by means of this particular phenomenon, 

 limited though it is to certain varieties of cell. This 

 narrow view leaves out of consideration reactions which 

 are, at least as constantly as " phagocytosis," a part of the 

 inflammation process, and of immunity and resistance. 

 Furthermore, strangely enough it overlooks altogether the 

 more general and more potent extracellular method of 

 digestion in which the cells attack food and prey in 

 their immediate environment, dissolving these without in- 

 cepting any particles of them. This is the process which 

 reaches its extreme perfection in the secretory epithelia of 

 the various special digestive glands, such as the salivary 

 and intestinal ; but it is recognisable in unicellular or- 

 ganisms, and, as above mentioned, in the coarsely granular 

 oxyphil leucocyte. 



I should extend the length of the present article too 

 greatly were I to attempt here any account of the various 

 forms and grades of " leucocytosis," that is to say, of 

 numerical increase of the wandering cells collected in a 

 part. Could we include such a description within this 



