98 PATHOGENIC BACTERIA. 



has brought forth numerous evidences. For example, 

 the varying action of staining reagents upon bacteria has 

 been made use of to demonstrate retrogressive changes 

 in the bacteria. In the frog's leukocyte Metschnikoff 

 thought the loss of affinity of the bacteria for vesuvin 

 might indicate its progressive dissolution, and in the 

 giant cells, formed in the liver of the "Zieselmaus" he 

 was able to show that the tubercle bacilli they contained 

 were surrounded by a halo which he thought consisted 

 of softening bacterial-cell protoplasm, but which Baum- 

 garten thought might equally well be looked upon as 

 softening cell protoplasm upon which the tubercle bacil- 

 lus was operating destructively. The experiments that 

 have been made with non-pathogenic bacteria seem to 

 indicate that the destructive processes of the cells are not 

 very rapid, for Wyssokowitsch found that spores of Bacil- 

 lus subtilis remained alive in the spleen for three months. 



The observations that led to Metschnikoff' s theory of 

 phagocytosis occurred during the period in which the bac- 

 teria themselves, rather than their products, were looked 

 upon as the cause of disease. As knowledge has accumu- 

 lated, it has been found that animals which are immune 

 to disease-producing bacteria are also immune to their 

 filtered toxic products. This kind of immunity certainly 

 cannot depend upon phagocytosis, but upon some more 

 intricate process, and the theory as originally propounded 

 becomes untenable. As Muir and Ritchie 1 point out, 

 "even if it were consistent with facts, it only removes the 

 property of immunity a step further back — namely, to 

 the phagocytes." " The phenomena of phagocytosis so 

 admirably demonstrated by Metschnikoff may be re- 

 garded as the result of immunity, but cannot be ac- 

 cepted as its cause." 



With the development and progress of knowledge upon 

 the subject Metschnikoff has never relinquished his orig- 

 inal idea that the leukocytes are the essential agents, 

 though the phenomena of immunization to toxins made 



1 Manual of Bacteriology, Edinburgh and London, 1897. 



