THE MOVEMENTS AND REACTIONS OF AMCEBA. 225 



GENERAL CONCLUSION. 



Putting all our results together, we must conclude that the move- 

 ments and reactions of Amoeba have as yet by no means been resolved 

 into their physical components. Amoeba is a drop of fluid which 

 moves in its usual locomotion in much the same way as inorganic drops 

 move under the influence of similarly directed forces. But what these 

 forces are is by no means clear. When we take into considera- 

 tion the currents as they actually exist, local decrease in surface tension 

 breaks down completely as an explanation for the locomotion and other 

 movements. The locomotion taken by itself might be explained as due 

 to the adhesion of the fluid protoplasm to solids, taken in connection 

 with the surface tension of the fluid, but this explanation fails when we 

 consider the formation of free pseudopodia, and discover that all the 

 processes concerned in locomotion can take place without adhesion to 

 the substratum. 



For the reactions to stimuli we find a parallel condition of affairs. 

 The currents in the protoplasm in the positive and negative reactions 

 are not similar to those produced in the attraction or repulsion of drops 

 of fluid by the direct action of external agents. Therefore we cannot 

 consider these reactions as due to the increase or decrease of surface 

 tension* produced by the direct (or even indirect) action of the external 

 agents. The taking and choice of food cannot be physically explained 

 in any general way by the physical adherence of the protoplasm as a 

 substance to the food as a substance, for food is taken in many cases 

 (usually, in some species) where it is demonstrable that no such 

 adherence exists. 



While we must agree that Amoeba, as a drop of fluid, is a marvel- 

 lously simple organism, we are compelled, I believe, to hold that it has 

 many traits which are comparable to the " reflexes" or "habits" of 

 higher organisms. f We may, perhaps, have faith that such traits are 



* It should be pointed out that this and other statements concerning surface 

 tension in Amoeba apply to the tension of the actual body surface, comparing 

 Amoeba thus to a drop of simple fluid. This is the basis on which rest the pre- 

 vailing theories that would explain the movements of Amoeba by surface tension. 

 It is these theories which I desired to test. There remains untouched, of course, 

 the possibility that the movements of all sorts of protoplasmic masses may be 

 explained by changes in the surface tension of the meshes of Biitschli's honey- 

 comb structure, in the manner indicated by Biitschli (1892, p. 208). But this is 

 at present merely a hypothesis, not worked out and not controllable by observa- 

 tion. To attempt to maintain it for Amoeba would be to relegate the movements 

 of this animal to the same obscure category as the movements of cilia and of 

 muscles, possibly a correct proceeding, but removing the matter at present from 

 the field of experimental observation. 



t See the next division of this paper, where this point is developed. 



