244 CHEMOTAXIS 



by which the phagocytes are brought into contact with the 

 bacteria in other words, of chemotaxis. This is a phenomenon 

 which is displayed by almost all motile and unicellular organisms, 

 whether animal or vegetable, and by the leucocytes of the higher 

 animals, and manifests itself in a movement of the organism in 

 response to a chemical stimulus. To take an example from the 

 bacteria : if a capillary tube containing a solution of meat-extract 

 be placed in a watery emulsion of B. coli or B. typhosus, the 

 bacteria will be seen to group themselves round the mouth of the 

 tube, which they ultimately enter. This is an example of positive 

 chemotaxis, the bacteria being attracted by the extractives, which, 

 indeed, they utilize as food. On the other hand, bacteria will 

 tend to remove themselves from an area from which alcohol and 

 similar substances are diffusing : this is negative chemotaxis. As 

 a general rule, we may say that motile organisms tend to be 

 attracted into a region rich in useful and nutritious substances, 

 and are repelled from injurious ones, but this is not invariably 

 true in the artificial conditions of experiment. An organism may 

 be lured by a useful substance into a region where there is a 

 sufficient quantity of an injurious one to kill it. 



In cases where leucocytes make their way into a tissue infected 

 with bacteria it must be, therefore, because the latter give off a 

 substance which has a positive chemotactic action on them. This 

 action may readily be shown by experiment, and the easiest way 

 is to work with the lymph of a cold-blooded animal, since in this 

 way all difficulties connected with a warm stage are avoided. If, 

 for instance, we take a few anthrax bacilli and place them on a 

 slide, add a drop of frog's lymph (from the dorsal lymph sac), and 

 apply a cover-glass, the leucocytes can be easily seen to crawl 

 actively up to the bacilli. To this experiment (Kanthack's) we 

 shall have to recur. 



In nearly all cases we find that leucocytes are attracted in large 

 numbers into the area in which the bacteria are situated i.e., 

 nearly all bacteria give off substances which are positively 

 chemotactic for leucocytes. In a few cases, however, this appears 

 at first sight not to happen, for when tissues which are infected 

 with very virulent bacteria are examined microscopically, they 

 are often extremely poor in leucocytes. As an example, we may 

 take any example of acute spreading gangrene of bacterial origin. 

 But, as Kanthack pointed out, these are not necessarily examples 

 of negative chemotaxis, and it is quite probable that the paucity 



