72 AMEBOID MOVEMENT 



point of a drop of fluid in such a way as to cause particles on the 

 surface to move toward that point. The principle underlying 

 the movement of the surface film in both cases is however exactly 

 the same; so, although it would be more desirable to compare 

 the surface movements in a drop of fluid in which the surface 

 tension is increased at some point, because this is what happens 

 in an ameba during locomotion, we shall nevertheless find it nec- 

 essary to consider a drop of fluid in which the surface tension has 

 been lowered. The application of the illustration is readily made. 



When the surface tension of a drop of fluid is lowered by 

 bringing into contact with it some other substance that possesses 

 this power, the surface rushes away at great speed in all directions 

 from the point where the tension is lowered, because usually the 

 tension is reduced very considerably. In this surface movement it 

 is found that new surface is made where the tension is lowered 

 and old surface is destroyed, that is, pulled into the interior 

 over a large part of the surface opposite to where the tension is 

 lowered. The speed of the surface movement is most rapid near 

 the point where the tension is lowered and becomes gradually 

 slower as the opposite side of the drop is approached, where there 

 is no movement. This variation in speed of the moving surface 

 seems to be due largely to the small area in which the tension is 

 lowered as compared with the whole surface of the drop. 



In the ameba the conditions are reversed. The surface layer 

 moves toward a point with increasing speed, instead of away 

 from a point. In both the ameba and the drop the greatest speed 

 is attained near the small area where the change in surface tension 

 occurs. 



The behavior of large and heavy particles on the surface of a 

 drop of fluid and on an ameba are similar. A heavy particle as 

 of sand, or a small glass rod, laid on the ameba, is not moved 

 by the surface layer. It forms an island of suface matter around 

 which the moving surface layer flows. Precisely the same thing 

 happens in surface layer movements in inanimate fluids. 



Again in point of thinness there is no disagreement so far as 

 microscopic observation goes. Neither the surface film on an 

 ameba nor the surface film on a fluid can be directly observed 

 microscopically to be different from the fluid below it. The sur~ 



