The behavior of large and heavy particles on the surface of a drop of fluid and on an ameba are similar. A heavy particle as of sand, or a small glass rod, laid on the ameba, is not moved by the surface layer. It forms an island of surface matter around which the moving surface layer flows. Precisely the same thing happens in surface layer movements in inanimate fluids. Again in point of thinness there is no disagreement so far as microscopic observation goes. Neither the surface film on an ameba nor the surface film on a fluid can be directly observed microscopically to be different from the fluid below it. The surface 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 surface 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 ingredients 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.