2o6 THE BEHAVIOR OF I.OWEK ORGANISMS. 



Now, when the interattraction of the particles at a certain region of 

 the surface of a mass of fluid is decreased, the pressure inward and the 

 tension along the surface are decreased. As the pressure remains the 

 same elsewhere, fluid tends to be pressed out at the point where the 

 pressure is lowered ; thus a projection may be formed here. As the 

 tension remains the same elsewhere, the remainder of the surface of 

 the drop pulls harder than that of the region under consideration ; 

 hence it pulls the surface of the fluid away from the region where the 

 tension is lowered. The efl'ect is similar to that which would be 

 produced if one portion of a stretched sheet of India rubber were 

 weakened or cut ; the remainder of the sheet would pull away from this 

 region. Thus there are produced the currents characteristic of such a 

 drop of fluid — an axial current toward the region of lowered tension, 

 surface currents away from the region of lowered tension (Fig. 34). 

 An increase in the tension at the opposite side would produce exactly 

 the same currents, as Rhumbler (189S, p. iSS) has set forth, the axial 

 current being always toward the region of lowest tension, the surface 

 currents in the opposite direction. 



In the moving Amceba, as we have seen, the currents are by no 

 means of this character. The axial current and the surface current are 

 congruent, and both are in the direction of locomotion. Such move- 

 ment could not be produced by a local decrease in the surface tension 

 of some part of the body surface. 



The formation of pseudopodia is, as we have seen, essentially the 

 same process as the forward movement at the anterior end of the 

 Amoeba. On the upper surface of a pseudopodium that is in contact 

 with the substratum there is a forward movement, so that particles 

 clinging to the upper surface are carried over the tip ; the currents 

 which must result from a decrease in surface tension are not present. 

 On the contrary, there is a current on the surface in the opposite direc- 

 tion from that required. The formation of such a pseudopodium can 

 not, then, be due to a local decrease in surface tension. 



The same is true, essentially, when a pseudopodium is sent out into 

 the water, not coming in contact with a surface. In such a case, as 

 we have seen, the entire surface moves outward, in the same direction 

 as the tip ; there is no such backward movement as the theory requires. 



Altogether, it is clear that the supposed resemblance between the 

 movements and internal cui'rents of AmcEba and those of a drop of fluid 

 moving as a result of a local increase or decrease of surface tension 

 does not exist. We must conclude that the movements of Amoeba are 

 not due to local changes in surface tension. 



One might be tempted to inquire whether the movement of Amoeba 

 could not be explained by considering separately the action of the two 



