164 PROTOPLASM 



pressure of the cell would then force a protrusion out at the point 

 where a weakening in the tension of the membrane has occurred, 

 just as a weak spot in a toy balloon gives rise to a protrusion 

 there. In such a way may a pseudopod be formed. Advance 

 would take place as follows: A pseudopod protrudes owing to 

 reduction in surface tension and clings to the substratum ; increase 

 in tension of the membrane now draws up the rear portion of the 

 amoeba; the process is then repeated. Berthold (1886) was 

 the first to advance such a theory, but Jennings later came to the 



Fig. 97. — The severance of a Paramoecium by Amoeba. {From S. O. Mast and 



F. M. Root.) 



conclusion that the locomotion of Amoeba is demonstrably not 

 due to a local decrease in surface tension on the side toward 

 which the animal is moving. 



Another process ascribed to surface-tension forces is that of the 

 ingestion or engulfing of food by Amoeba. When by chance 

 Amoeba meets its prey, it surrounds it with two pseudopods. 

 The victim thus caught may be taken in whole and pocketed 

 in a spontaneously formed food vacuole, or just a bite may be 

 taken out of it (Fig, 97). The unicellular animal Paramoe- 

 cium, on which Amoeba feeds, maintains a permanent shape. It 

 must, therefore, possess a more rigid pellicle than does Amoeba, 

 which is always changing form. How then, is it physically 

 possible for Amoeba with its jelly arms to pinch a firm para- 

 moecium in two? Surface tension was presumed to be the force 

 involved, but Mast and Root made some calculations. They 

 found that to cut a paramoecium in half with a fine glass fiber 

 requires a pressure of approximately 9 mg. A reduction in 



