Immunity against pathogenic micro-organisms 137 



nucleus, however, is sometimes divided into lobes. It is to be noted 

 that in the perch these are the sole representatives of the motile 

 phagocytes, and that in this fish not only the eosinophile but every 

 other variety of granular leucocyte is completely wanting. In the 

 gudgeon, in addition to haemomacrophages, some microphages whose 

 protoplasm stains faintly with acid aniline colours are met with. 

 These facts will be useful to us when we come to study the part 

 played by phagocytes in immunity from a general point of view. 



Another class of cold-blooded animal, the Amphibia, has been 

 much more frequently studied from the point of view of infection 

 and immunity. The frog, an animal so convenient for many physio- 

 logical and pathological researches, has been much employed for 

 the study of immunity against pathogenic micro-organisms. Quite 

 a literature, which has been excellently summarised in the memoir of 

 Mesnil already cited, and to which we shall have occasion to return 

 more than once, has been accumulated on the subject. 



The immunity of frogs against the anthrax bacillus was early 

 demonstrated and studied in Robert Koch's celebrated memoir 1 

 on anthrax. This observer, after injecting an emulsion of anthrax 

 spleen into the lymph sac of the frog, recovered the bacilli from 

 the interior of round cells which burst readily when transported 

 into water. Koch, accepting the view then generally held, thought 

 that the bacilli found a favourable culture medium in the contents 

 of certain cells, but that, in spite of this, the frog was capable of 

 manifesting a real immunity against anthrax. Gibier 2 made the [^6] 

 interesting discovery that frogs when subjected to the influence 

 of high temperature (about 37 C.) lose their natural immunity and 

 readily contract fatal anthrax. 



Since that time the mechanism by which the organism of the frog 

 secures immunity against the anthrax bacillus has repeatedly been 

 studied. In a memoir which appeared in 1884 3 I insisted that the 

 principal part played in this immunity belonged to the phagocytes 

 which devour the injected bacteria and subject them to intra- 

 cellular digestion. The round cells described by Koch are nothing 

 but the leucocytes of the lymph sac which have seized upon the 

 anthrax bacilli. These bacilli instead of thriving in the cell contents 

 find there a very unfavourable medium, and perish at the end of 



1 Cohn's " Beitrage zur Biologie der Pflauzen," Breslau, 1870, Bd. II, S. 300. 



2 Compt. rend. Acad. d. *c., Paris, 1882, t xciv, p. 1605. 

 8 Virchmcs Archie, Berlin, 1884, Bd. xcvn, S. 502. 



