176 THE BEHAVIOR OF LOWER ORGANISMS. 



the rod and straightened out (). On being released it at once swung 

 back to its original form and position at a. 



An Amoeba had many long, slender, free pseudopodia standing out 

 radially from the body. These could be pushed repeatedly to one side 

 or the other, or bent at a marked angle. In every case they returned 

 at once to the radial position. 



An indefinite number of such experiments could be detailed. They 

 show clearly that Amoeba has, to a certain degree at least, one of the 

 most distinctive properties of solids, a tendency to resist deformation of 

 shape, and to restore the shape when changed. It will be observed 

 that the cases are not such as can be accounted for on the assumption 

 that Amoeba is a simple fluid mass which tends to take a certain form 

 in accordance with the principle of least surfaces. A small sphere of 

 fluid when deformed returns to its original shape in conformity with 

 the principle just mentioned. But such returns to the original form as 



are illustrated in Fig. 59 

 and Fig. 61, for example, 

 are not required by the 

 principle of least surfaces 

 so long as the Amoeba is 

 considered a simple fluid 

 mass. 



FlG ' 6l< * On the other hand, if 



we consider Amoeba not as a simple fluid, but as a fluid mass of 

 a foamlike or alveolar structure, composed of a tense meshwork of one 

 fluid enclosing minute drops of another, then the results above set 

 forth might be explained without assuming that the protoplasm has 

 in any part passed from a liquid to a solid state. This follows 

 from the considerations and experiments recently set forth by Rhum- 

 bler in a most important and suggestive paper (1902). Rhutnbler 

 shows that a fluid mass having alveolar structure must react to tran- 

 sient pressure from without like an elastic body ; in other words, 

 that it must have elasticity of form. The results which I have set 

 forth above might almost seem, then, to have been predicted in Rhum- 

 bler's summing up : " Transient tensions or pressures produce an 

 elastic reaction of the cell body; longer action, on the other hand, 

 produces a plastic reaction " (/. c.,p. 371). For the detailed demonstra- 

 tion of this principle the reader must be referred to the original paper 

 of Rhumbler. It will suffice to note here that the result is due to the 

 fact that deformations of the body as a whole produce deformations of 

 the alveoli, and that the surface tension of the alveolar walls tends to 



* FIG. 61. A large, curved pseudopodium, a, is straightened out into the posi- 

 tion b with a rod. On being released it at once returns to the position . 



