METABOLIC GRADIENTS IN AMOEBA 73 



finally resumes locomotion. Figures 9 to 11 illustrate these 

 reactions which may be regarded as avoiding reactions, in which 

 the stimulus is completely directive. 



Now it is necessary in order to imitate the action of the cya- 

 nide to use a non-directive stimulus, and I have attempted to 

 accomplish this by thrusting the needle point through the center 

 of the amoeba. The result of this procedure completely aston- 

 ished me. I found that after the usual powerful contraction of 

 the anterior end, the new pseudopodia almost invariably appear 

 at the posterior end, or near the posterior end, that is, in the 

 posterior half of the animal. I have repeated this experiment 

 innumerable times, and in all cases where the stimulus is suc- 

 cessfully applied to the center, the new pseudopodia have never 

 been seen to arise from the anterior half but always from the 

 posterior half of the amoeba. Figures 12 to 14 illustrate this 

 reaction. The experiment is best performed on individuals with 

 a single large pseudopodium (so-called limax type) since the 

 needle can be more accurately placed. The hay amoeba (fig. 13) 

 very frequently takes this form, while the wheat amoeba (figs. 

 12 and 14) tends to spread at its anterior end in a fan-like 

 manner. 



I have designated this reaction the shock reaction, as I con- 

 ceive it to be a reaction to a strong but non-directive stimulus 

 while the term avoiding implies that the localization of the, 

 stimulus has a directive effect on the response. Although it 

 appears to be a fixed reaction, something like the reflexes of 

 the higher forms, it is probably only a variation of the avoiding 

 reaction, since Jennings has found that Paramaecium when sur- 

 rounded on all sides by a stimulating chemical repeatedly gives 

 the avoiding reaction. 9 I suspect, however, that it would be 

 more correct to say that the avoiding reaction is a variation of 

 the shock reaction. The distinction, as I see it, between the 

 two is one of degree only, namely, in the avoiding reaction, the 



9 1 have found that amoeba gives the shock reaction when alcohol {about 50 

 per cent strength) is dropped upon it, and Mr. J. W. MacArthur of this laboratory 

 has observed that the shock reaction is the initial effect of exposure of the amoeba 

 to calcium chloride solution. 



