CRITICISM OF BERTHOLD 295 



of AmoebjE, and to class the latter as adhering drops of this 

 kind, as Berthold does. His hypothesis starts, as has been 

 said, from the view that the Amoeba adheres to the solid 

 substratum of the underlying surface and, being a fluid drop, 

 is subject to the above-mentioned conditions, in accordance 

 with which its marginal angle must also remain constant 

 as long as the chemical composition of the protoplasm is 

 constant. 



Now if, at a point of the edge of the Amoeba, a chemical 

 alteration takes place, whereby a lowering of the surface 

 tension (ajj) between the protoplasm and the underlying 

 surface is brought about, or, as Berthold usually expresses 

 it, the adhesion between the substance of the Amoeba and 

 the substratum is increased, there then results an extension 

 of this margin, until the marginal angle is sufficiently 

 diminished, to compensate for this change of the surface 

 tension or of the adhesion. If we have already reason 

 to doubt this explanation, on account of Quincke's ex- 

 perience to the effect that surface tension has no 

 influence upon adhering water drops, there are in 

 addition a considerable number of further points which, in 

 my opinion, are evidence against it. In the first place, 

 I consider it incorrect to suppose that Amoebse really 

 adhere to the solid substratum. I do not entirely dispute 

 the fact that local adhesions at the hinder end, or occasion- 

 ally also in the pseudopodia during their retraction, may 

 come - under observation. On the other hand, I consider 

 it certain that an extensive adhesion is absent. Any one 

 who has been freqi;iently occupied with Amcebee knows that 

 even very feeble currents of water are usually sufficient to 

 wash them away from the surface on which they are creep- 

 ing, while really powerful forces would in any case be 

 necessary for this, if an actual adhesion existed. In 

 addition to this there is the fact that some Amoebse, which 

 certainly do not adhere, but swim freely in the water, 

 develop pseudopodia, and their power of changing ' their 

 shape is in no way impaired.^ 



1 A very suitable object upon which to convince oneself of the non- 

 adherence of very motile Amoebfe is Pelomyxa. With some good motile 



