AMEBOID MOVEMENT 73 



face layer is, as is generally believed, of molecular dimensions, 

 and its thickness is beyond the limits of vision. Unless some 

 special means is discovered therefore for making visible the sur- 

 face film, such as a process of staining, it may be impossible to 

 ascertain its ultimate structure directly, for it overlies a mass of 

 heterogeneous fluid whose composition is constantly changing. 



It seems to follow from what is observed of the surface tension 

 layers of the fluids of physics that such layers must be of the 

 same constitution as the body of the fluid over which the layer 

 is formed, although, as is well known, the proportion of the in- 

 gredients in the surface layer is different from that in the body 

 of the fluid. Now since the resemblance between the surface 

 layer of an ameba and a surface layer on a drop of fluid has thus 

 far been found to be complete, it is pertinent at this point to 

 discuss Gruber's ('12) suggestion that the movement of particles 

 forward on an ameba is due to the forward movement of an inert 

 layer of mucus or gelatinous material secreted by the ameba. 



To begin with, observation does not support Gruber's sugges- 

 tion. No such layer can be seen. Such a layer, since it is shown 

 to persist for several minutes at least, should remain after an 

 ameba bursts, under experimental conditions, but no such re- 

 mains can be seen. Its existence should be demonstrable by the 

 use of dyes, but the evidence is negative. Indeed there is not any 

 direct evidence that can be brought in support of the suggestion 

 that this surface layer is gelatinous in composition. Moreover, 

 as we have seen, the layer on the ameba that carries particles 

 forward seems to be destroyed at the anterior end, for in what 

 other way would particles remain at the anterior end after being 

 brought there ? But the supposition that a gelatinous layer might 

 be drawn into the interior at the anterior end is also negatived 

 by observation, for no very small particles clinging to or imbedded 

 in the surface substance are ever drawn into the ameba, as would 

 almost certainly be the case if the substance composing the layer 

 were gelatinous. And as to supposing that this layer, if gelatinous, 

 might behave essentially as a surface tension layer and therefore 

 be drawn in at the anterior end of the ameba, this is contrary to 

 the experience of physics; for the physical nature of the ameba 

 would make it impossible for the ameba to have a surface layer 



