AMEBOID MOVEMENT 75 



principle of Willard Gibbs, will be found in higher concentration 

 in the surface film than in the body of the ameba. 



Perhaps the most important question that arises in connection 

 with the surface layer of the ameba is : What causes it to move 

 in the manner described? But we can do little more than ask 

 the question. It has been seen that the surface film moves toward 

 an area of increased tension rather than from an area where the 

 tension has been lowered. However, since we are completely in 

 the dark respecting the composition of the surface layer or of 

 the fluid parts of the ameba, it is exceedingly hazardous to venture 

 an explanation. If the surface layer should have its tension 

 lowered by a concentration of lipoids in it, we would be faced 

 by the necessity of explaining their removal at the anterior end. 

 If we turn to electrical causes we 'meet again with great dif- 

 ficulties. An ameba moves with the electric current, when a cur- 

 rent is passed through the water. The surface layer under these 

 conditions behaves normally, as may be inferred from Jennings' 

 ('04) figure on page 198. That is, the current controls the direc- 

 tion of the movement of the ameba, with the current leaving the 

 ameba at the point of highest surface tension. This is contrary 

 to the action of the mercuric capillary electrometer, in which the 

 mercury column also moves with the current, but because of low- 

 ered surface tension where the current leaves the mercury. The 

 conditions surrounding these cases are so different however, that 

 very little can be gained by setting them in contrast to each other. 



