74 AGE, GROWTH, AND DEATH 



are no longer restrained, and they at once do their 

 work. 



There is another, but comparatively rare, mode of 

 cell destruction. The cells break up into separate 

 fragments, 3 which are then dissolved by chemical 

 means and disappear, by the method of histolysis 

 above described, or else are devoured by the cells 

 to which reference was made in the first lecture and 

 which are known by the name of phagocytes, and to 

 which Metchnikoff has attributed so great an impor- 

 tance. It is unquestionable that phagocytes do eat 

 up fragments of cells and of tissues, and may even 

 attack whole cells. But to me it seems probable that 

 their role is entirely secondary. They do not cause 

 the death of cells, but they feed presumably only 

 upon cells which are already dead or at least dying. 

 Their activity is to be regarded, so far as the prob- 

 lem of the death of cells is concerned, not as in- 

 dicating the cause of death, but as a phenomenon for 

 the display of which the death of the cell offers an 

 opportunity. A word of caution ! Let me state 

 explicitly that the death of cells does not depend 

 always upon their completing the cytomorphic cycle. 

 Death may befall a young cell just as it may befall a 

 young child. I think it probable in all such cases, 



" The best known case of fragmentation is that of the red-blood corpuscles. 

 Vast numbers of them are constantly destroyed at the close of their cytomor- 

 phosis by this process, which has been studied by numerous investigators 

 chiefly in the spleen and the liver. Another noteworthy illustration of this 

 method of cell destruction was discovered by Ranvier (" Des clasmatocytes," 

 Archives I'anatomie microsc., iii., 122-139) among wandering cells which 

 occur in the connective tissue of mammals. 



