74 AMEBOID MOVEMENT 



of gelatinous matter. There do not seem to exist any grounds 

 therefore for supposing that the outermost layer of an ameba, 

 .the layer that carries particles as described in the preceding pages, 

 can consist of an inert substance as Gruber suggests. 4 



From these considerations, then it appears that all the evidence 

 available, both direct and indirect, points to the conclusion that 

 the behavior of the surface layer on the ameba resembles in gen- 

 eral and in detail the behavior of a surface tension layer in an in- 

 ert drop of fluid, and that we must regard the surface layer on the 

 ameba as a true surface tension layer. This layer is therefore a 

 dynamic layer, containing free energy, and capable of performing 

 work. It is physiologically distinct from ectoplasm, as ectoplasm 

 is distinct physiologically from endoplasm. But the distinctive 

 properties which the surface layer possesses are functions of its 

 position. These properties clearly indicate that its constitution 

 is protoplasmic, corresponding to the fluid parts of the internal 

 protoplasm. 



The surface layer of the ameba is probably identical with what 

 is commonly called the plasma membrane or semi-permeable 

 membrane as postulated by Overton ('07). The peculiar struc- 

 ture supposed to be possessed by plasma membranes are held to be 

 due chiefly to surface forces. The fact that the surface layer 

 of the ameba is continually being destroyed and re-created during 

 locomotion does not support the view that the plasma membrane 

 is of inert composition, as for example, lipoidal, as has been 

 suggested. The observations, on the contrary, confirm Hober's 

 {'n) view that the plasma membranes generally are living struc- 

 tures. But it may be regarded as certain that if lipoids are present 

 in the protoplasm of the ameba, these substances, according to the 



4 It is possible that Gruber was led to suggest a gelatinous composition 

 for the layer in question on the strength of assertions made by several 

 writers that amebas secrete mucus. It is true that amebas may be dis- 

 placed by threads of mucus hanging to glass needles which has collected 

 on the needles while manipulating the amebas in the culture medium, 

 but that is not to be taken as evidence that the mucus is secreted by the 

 amebas. Ameba cultures are always full of gelatinous material formed 

 by bacteria. I have not thus far been able to convince myself that amebas 

 actually secrete mucus.. 



