250 A MANUAL OF BACTERIOLOGY 



cinated against anthrax phagocytosis is very active. 

 Moreover, in an animal refractory to anthrax, such as 

 the frog, anthrax bacilli grow and multiply if they be 

 enclosed in paper or collodion sacs, so as to prevent the 

 access of the phagocytes. 



Phagocytosis in vitro, and probably also in the normal 

 body, is extraordinarily active, so that it might be 

 expected always to be sufficient to deal with any number 

 of bacteria that might be introduced. If, however, the 

 bacteria be virulent, negative chemotaxis will occur. 

 Moreover, the presence of substances which render the 

 bacteria phagocytosable, " opsonins," is necessary, and 

 it seems likely that the amount of opsonin becomes 

 diminished in infection (see p. 257). 



Metchnikoff admits that the destruction of bacteria in 

 phagocytosis is brought about by chemical bacteriolytic 

 substances, which he terms " cytases," and which he 

 regards as being derived from the leucocytes, and as 

 identical with the alexins. He believes that there are two 

 kinds of cytases, one " macrocytase," obtainable from 

 tissues, such as the spleen and lymph -glands, rich in 

 macrophages, which acts specially on elements of animal 

 origin, the other " microcytase," derived from the micro- 

 phages, and which acts principally on micro-organisms. 

 He considers the alexic action to be of the nature of a 

 digestive process (but this is doubtful), and as regards 

 the complex nature of a cytolytic serum, which contains 

 amboceptor and complement, believes that the ambo- 

 ceptor is formed within the macrophages in intra-cellular 

 digestion, and that a portion of it escapes from them into 

 the serum. All the facts point to the leucocytes and 

 leucocytic tissues being the great defensive mechanisms 

 against parasitic invasion, either by the production of 

 alexins, or of bacteriolysins, or by phagocytosis, or prob- 

 ably by a combination of these (the " cellulo -humoral " 



