How is the difference in the speed of movement of the surface layer between _sphaeronucleosus_ and _discoides_ to be explained? There are no ridges to retard the movement of particles in _discoides_, while there are ridges in _sphaeronucleosus_, where the particles move on the average twice as fast as on _discoides_. In the first place the advancing edge, the edge where ectoplasm is being made, is proportionately much wider in _sphaeronucleosus_ than in _discoides_ as compared with the amount of surface back of it. Figures 23 and 24 show that the rate of movement of the surface film is directly proportional to the amount of new ectoplasm forming. In the second place, the greater part of the under surface in the forward half of _sphaeronucleosus_ is attached to the substrate, so that the surface layer which flows toward the anterior end is derived almost wholly from the upper surface; while in _discoides_ the whole surface in free pseudopods, and nearly the whole surface in attached amebas (cf. Dellinger’s observations described on p. 56) possesses mobile surface protoplasm. Observation of moving particles on these amebas proves this. Then again, the anterior edge of a _sphaeronucleosus_ is not attached at the points farthest advanced, but the point of attachment is some distance back, as indicated in figure 20. The effect of this is to increase the amount of forming ectoplasm in proportion to the surface of the ameba from which surface protoplasm may be drawn. Still one other factor must be considered. As is well known _sphaeronucleosus, verrucosa_ and their congeners possess longitudinal ridges on the upper surface which consist of ectoplasm, covered of course by the surface film. These ridges are formed near the anterior edge, not by wrinkling, but by the construction of new ectoplasm. Once formed, they remain until the ameba, so to speak, flows out from under them. That is, the ridges undergo comparatively slight changes until changed back into endoplasm at the posterior end of the ameba. As the ameba flows ahead the ridges are of course continually being added to or lengthened, by the conversion of some endoplasm into ectoplasm. The ridges may thus retain their identity for a long time although the substance composing them is changed every time the ameba moves the length of its body. It is clear, therefore, that there is more ectoplasm formed at the anterior end of a _sphaeronucleosus_ than would be the case were the upper surface of the ameba plane; and the conclusion therefore is obvious that the formation of ridges, occurring as it does, chiefly at the anterior end, serves further to accelerate the forward movement of the surface film.