in 



LIVING MATTER 



former consists in the active approach of the micro-organisms to 

 the source of the chemical stimulus, as if attracted by it ; the 

 second consists in the opposite phenom- 

 enon, i.e. active withdrawal from the seat 

 of the stimulus, as if it exerted some 

 repulsive action. 



A given solution may be an energetic 

 chemotactic stimulus for one organism, 

 and weak 'for another. The efficiency of 

 the stimulus depends on its chemical 

 constitution ; potash, e.g., is active in 

 combination with one acid and not with 

 another. Certain poisons (sodium salicyl- 

 ate, morphia) in weak solutions exert an 

 attractive action, in concentrated forms 

 a repulsive action. Some substances 

 (alcohol, alkalies, free acids) always have 

 a repellent action, i.e. they exert negative 

 chemotaxis. 



The method adopted by Pfeffer in 

 studying chemotaxis is very simple : he 

 merely immerses in the water which con- 

 tains the microbes a capillary glass tube 

 filled with the solution to be investigated, 

 and closed at one end. If the microbes 

 penetrate into the tube, there is positive 

 chemotaxis ; if they move away, there is 

 negative chemotaxis. If, e.g., a O'Oo per 

 cent solution of malic acid is introduced 

 into the capillary tube, the open end of 

 which dips into a drop of fluid containing 

 the spermatozoids of Ferns, the malic acid 

 will scarcely have begun to diffuse in the 

 drop when the spermatozoids move towards 

 the entrance of the tube and crowd into 

 it. The same thing may be seen with a 

 much weaker solution (O'OOl per cent) of 

 malic acid. The movements of the sper- 

 matozoids must be directed by the differ- 

 ence in concentration of the acid which 

 is in contact with the different parts of 

 their body. When the concentration of 

 the acid diffused in the drop becomes the 

 same at every point, it can no longer 



exercise any directive action upon the movements of the sper- 

 matozoids. 



Leber, Massart and Bordet, Metschnikoff and others discovered 



FIG. 19. Positive cliemotaxis of 

 leucocytes in presence of Maphy- 

 lococcu&pyogenes allius. (Massart. ) 

 Capillary glass tube (magnified 

 under the microscope), closed 

 at one end, and tilled with a 

 culture of Staphylococcus, to- 

 wards which the leucocytes 

 are streaming through the open 

 end of the tube. The observa- 

 tion is made after the capil- 

 lary tube has been introduced 

 into the peritoneal cavity of 

 the frog, or beneath the skin of 

 a rabbit, and kept there 10-12 

 hours. 



