THE ANIMAL A MACHINE 165 



strict ed by a force acting equally all around the drop, the 

 "surface tension." Now, the magnitude of this force is 

 diminished, if an interacting substance, such as alcohol, is 

 contained in the oil. If the alcohol is injected into the oil- 

 drop on one side, as described by Jennings, the diminution 

 of the contracting force must lead to a distortion of the 

 regular round shape of the drop. The contracting force 

 surrounding most of the surface of the drop will, of neces- 

 sity, squeeze out the oil at any place where the surface 

 tension is lowered. We see, therefore, how the formation 

 of "pseudopodia" in the oil-drop comes about. 



If the alcohol is not injected into the oil-drop but added 

 to the surrounding liquid, it will penetrate into the drop, 

 but at one point more than at the neighboring point, thus 

 causing a lowering of surface tension on one side and giving 

 rise to amoeboid movement- 

 Why should we assume that a different sort of force acts 

 in the Amoeba? There is no reason for such an assumption, 

 but in order to demonstrate this point beyond doubt, we 

 shall compare the movements of the oil-drop and of the 

 Amoeba with respect to many different details, as follows: 



A. Internal Streaming 



As described, the protrusion of pseudopodia in Amoeba 

 is accompanied by an internal streaming motion (a rapid 

 forward streaming in the center and a slower returning near 

 the margin). Exactly the same type of streaming is ob- 

 served in an oil-drop. (Fig. 64, turn to next page.) 



B. The Tortuous Path 



As the Amoeba slowly moves in the water by protrusion 

 of pseudopodia, its path is very tortuous, since the direction 

 of these protruding extensions varies from second to second. 

 Exactly the same is true for the oil-drop. This feature can 

 best be demonstrated with a drop of a heavy oil, or oil-like 



