THE MOVEMEVTS AND REACTIONS OF AMCEBA. 221 



Based similarly on a direct action of external agents is the theory of 

 amoeboid movements proposed by Verworn in 1891, and extended in 

 his paper on Die Bewegnng der lebendigen Substanz (1892) and in 

 the Allgemeine Physiologic. In its original form Verworn's theory 

 considers the movements and changes of form to be brought about 

 directly through chemical attraction (Verworn, 1891, p. 105), but in 

 later publications (1892, 1895) the effect of the chemical is considered, 

 as in Rhumbler's theory, to be that of increasing or decreasing the 

 surface tension. 



Thus it is evident that it must be the chemical affinity of certain parts of the 

 protoplasm for oxygen that decreases the surface tension in definite regions and 

 thus leads to the formation of pseudopodia. But it will be possible for the same 

 effect to be produced by other substances of the surrounding medium, if they 

 have chemical affinity for certain components of the protoplasm. In the case 

 where the substance acts from one side, this principle must lead to positive 

 chemotropism. (Verworn, 1895, p. 545.) 



It is evident that the method of movement of Amoeba, as described 

 in this paper, has an immediate bearing on the question of direct or 

 indirect action of external agents. If the action of an external agent 

 is to increase or decrease directly the surface tension, as set forth by 

 Rhumbler and Verworn, this effect must be shown in the characteris- 

 tic currents which appear in any fluid when the surface tension is thus 

 locally changed. In the case of negative chemotaxis we should have 

 an axial current away from the side affected, with surface currents 

 toward the chemical, as indicated in the figure given by Rhumbler 

 (1898, p. iSS). In positive chemotaxis both sets of currents should be 

 the reverse of that just indicated. 



In the account of the movements set forth by Biitschli and Rhumbler, 

 the currents agreed with the scheme for direct action above set forth. 

 But this account of the movements was erroneous, as we have seen. 

 The internal currents and the surface currents are forward, away from 

 the region stimulated, in a negative reaction ; toward the region stim- 

 ulated in a positive reaction ; the movement is of a rolling character. 

 There is thus no evidence that the action of the stimulus is to cause a 

 change in the surface tension of the parts directly affected ; on the con- 

 trary, the direction of the currents is quite inconsistent with this view.* 

 We must conclude, then, that the theory of the direct action of exter- 

 nal agents in causing or changing the movements of Amoeba is nega- 

 tived by the character of the movements produced ; these are not such as 

 would follow from the direct physical action of the agents in question. 



* A description of the forward surface currents in negative chemotaxis is given 

 on p. 143; in the reaction to a mechanical stimulus on p. 185; to the electrical 

 stimulus on p. 192 ; in a positive food reaction (chemical and mechanical stimuli ?) 

 on p. 198. 



