THE MOVEMENTS OF ORGANISMS 285 



At times small amebse are pursued by larger ones, the former 

 change their direction and their speed, the pursuer continues its 

 journey and catches its prey, which may again escape, and the 

 pursuit continues. All these processes are explained according to 

 L. RHUMBLER, without invoking a conscious intelligence and pur- 

 poseful movements, by the trail left behind by the pursued ameba, just 

 as the chloroform drop pursues the track of shellac mentioned above. 



Though the movements are so similar and the explanation by 

 changing surface tension is so clear, we are still forced to enquire 

 how the surface tension of amebse and leucocytes is changed. An- 

 alogy is quite absent in the character of the substances whose sur- 

 faces are in contact and in the physical process (solution of the shellac) 

 that takes place. It was assumed that substances which dimmish 

 surface tension (for instance, soaps, albuminates, L. MICHAELIS) form 

 at the point of motion and then break up again. Though a definite 

 demonstration has not been possible, I shall discuss an hypothesis of 

 L. HIRSCHFELD* which has much to recommend it in certain cases. 

 We know that an electric charge depresses the surface tension (see 

 p. 87) but the question is whether the development of an electrical 

 charge at any point of a mass of protoplasm is conceivable. Let 

 us consider the circumstances under which a bacterium approaches 

 an ameba that puts out a pseudopodium, envelops the bacterium 

 and draws it in. Between two electrodes, amebse migrate to the 

 cathode and bacteria to the anode. H ions diminish surface ten- 

 sion, causing the extension of pseudopodia as demonstrated by the 

 plentiful formation of pseudopodia upon fixation with osmic acid; 

 OH ions cause an increase of surface tension and a retraction of 

 pseudopodia. If we imagine a bacterium to be a negatively charged 

 particle which gives off H ions, by dissociation it will lower the surface 

 tension at the presenting point of the ameba and occasion the appear- 

 ance of pseudopodia. When the bacterium is surrounded, there is an 

 equalization of charge, the surface tension is raised and the pseudo- 

 podium is retracted with the bacterium. L. HIRSCHFELD attributes 

 the positive charge of amebae to the excretion of C0 2 . If the 

 metabolism of the ameba is impaired, the formation of CO 2 , and 

 with it the mobility of the ameba, are diminished. What occurs in 

 the case of amebae may be applied to the special case of phagocytosis. 

 It was the phenomena occurring in amebse that led ELIE METCHNI- 

 KOFF to his fundamental studies on phagocytes, scavenger cells. 

 Thus he names such white blood corpuscles as attack by taking up 

 and digesting microorganisms entering the blood stream. They are 

 the defending army of the organism, and according to E. METCHNI- 

 KOFF, the most important weapon in the fight against disease germs. 



