394 WM. A. KEPNER AND J. GRAHAM EDWARDS 



was always the smallest possible amount of water involved in 

 the formation of the food-vacuole. 



Schaeffer ('12) says: " Encircling seems to take place when- 

 ever the stimuli coming from a particle are not sufficiently 

 strong to produce movement directly toward the source of the 

 stimulus, nor weak enough to be ignored by the amoeba. But 

 these conditions would not be sufficient to produce encircling 

 such as we see in amoeba. Such conditions would doubtless 

 produce uncertain behavior, but it would not necessarily ex- 

 press itself in encircling the test object. There must, there- 

 fore, be another factor present in amoeba which is concerned 

 with encircling. This factor is a tendency to continue moving 

 forward after movement is once started. The amoeba seems 

 to acquire some sort of a momentum of reaction, which tends to 

 keep amoeba moving in more or less straight paths. Such a 

 tendency balanced against stimuli producing mild positive be- 

 havior would result in encircling movements as illustrated in 

 figures 2 and 3," page 63-64. 



It appears to us that an effort to apply physiological momen- 

 tum as a factor to explain encircling movement in Pelomyxa 

 would be futile. Because \vc have frequently seen a food- 

 vacuole well advanced in its formation, which after the prey 

 had escaped was reversed. Figure 13, A and B shows two 

 reactions in which the encircling movement cannot be explained 

 by physiological momentum. When reaction A set in, there 

 were three paramaecia involved. Before the reaction had 

 well begun the remote paramaecium retreated. When pseudo- 

 pods a and a' had been formed the latter striking the nearest 

 ciliate, caused it to retreat leaving only the middle paramaecium. 

 The withdrawal of the remoter paramaecium, no doubt, but 

 little modified the intensity of stimulus; but when the nearer 

 ciliate retreated the strongest source of stimulation must have 

 been removed. It was interesting, therefore, to note that though 

 the stimulus was weakened, the pseudopods continued to diverge 

 as they grew. Moreover, when pseudopod b' hit the remaining 

 paramaecium and caused it to retreat the further reduction of 

 the stimulation to zero resulted no longer in a divergence, but in 



