AMCEBOID MOVEMENT BERTHOLD 293 



further show how, above all, it comes about that this current 

 only attains development on one side, for, as we saw before, 

 every extension - current radiates out on all sides from 

 the place of its origin. Later on it will be a subject of 

 inquiry whether it is possible to find an explanation for the 

 one-sided nature of the rotational stream. For the rest, 

 Berthold has clearly recognised that the weakest point of his 

 explanation of the circulatory streamings consists in the fact 

 that in them true centres of extension, such as the 

 hypothesis postulates, are scarcely to be made out with 

 certainty. He seeks the cause of this fact in the thinness of 

 the protoplasmic lining of the wall, but I hardly think that 

 the matter is to be cleared up thus. 



It is curious, however, that Berthold is of the opinion 

 that the movements and streamings of Amoebae and plas- 

 modia do not depend on the same causes that produce 

 the streamings of the protoplasm of plant cells, although the 

 forces that are at work are the same in principle. He 

 believes that it is not extension -currents, depending on 

 local diminution of surface tension, and the forward 

 movements that appear in consequence, such as our 

 drops of oil or foam show under suitable conditions, 

 which form the cause of amoeboid movement, but that 

 the protoplasm of the Amoeba behaves in much the same 

 manner as a fluid which spreads out upon a solid 

 body. In order, therefore, to be able to understand 

 Berthold's view with regard to these processes, it is neces- 

 sary to pay a little attention to the conditions of the 

 spreading out of fluids upon solid bodies. Quincke, upon 

 whose views Berthold supports himself, has attempted in 

 1887 to give a theoretical foundation to the proposition 

 that the spreading out of fluids upon solid bodies is 

 governed by the same conditions which also determine the 

 spreading out of one fluid upon the surface of another, or 

 rather upon the limiting surface of two others. He holds 

 it admissible to assume that even at the limit between a 

 fluid and a solid body, in fact even at the surface of a solid 

 body, a surface tension exists, and that it is the ratio of the 

 magnitudes of the three surface tensions, i.e. of the surface 



