move toward that part of the anterior edge that is advancing most rapidly. Figures 17 and 18 illustrate this point. Figure 17 shows an ameba with two particles on its back, and with an unequally advancing anterior edge. Particle _a_ moved more rapidly than _b_ because: (1) it was moving away from a more rapidly receding posterior region; (2) the right anterior edge was advancing more rapidly than the left anterior edge; (3) the particle was nearer the anterior edge. The rapidly advancing right edge in stage 4 accounts for the veering of the particle _a_ to the right. The more rapid advance of _b_ from stage 3 to 5 is due to the remoteness of the anterior right edge, which, because of its nearness to particle a pulls on it to a much greater extent than on particle _b_. That is to say, when a particle lies somewhere _between_ two rapidly growing regions on the anterior edge, leading in different directions, that particle is attracted to the edge less rapidly than a particle lying immediately back of either advancing region. As may readily be observed each change in speed or direction of movement of the particle _b_ finds its explanation in the amount and location of ectoplasm formation at the time. Large particles like _a_ do not so readily reflect changes in the direction of pull of the surface layer. The rapid rate of movement of particle _a_--3.5 times as fast as the ameba--finds its explanation in an actively advancing anterior edge that was unusually wide. Particle _b_ moved at a slower rate, 2.7 to 1. It started from near the posterior edge where it moved comparatively slowly for a short distance. [Illustration: Figure 18. Illustrating the effect on the path of a particle attached to the surface film of an _Amoeba sphaeronucleosus_ when the ameba changes its direction of movement. From stages 3 to 5 the ameba veered to the right, also the particle. From stages 6 to 9 the ameba turned sharply to the left, and this change of direction was reflected in the movement of the particle. Length of the ameba, about 120 microns.] Figure 18 shows more pronounced changes in the direction taken by a particle attached to the back of an ameba. The change in direction at stage 6 was caused by a wave of ectoplasm thrown out at the left side, and cessation of movement at the anterior edge. At 7 a small wave was thrown out at the anterior edge and a large wave on the left. At stages 8 and 9 the direction of the particle was again a response to the waves of ectoplasm thrown out at the left anterior edge, which thus became the anterior end. [Illustration: Figure 19. Illustrating the rapid movement of the upper surface of an _Amoeba sphaeronucleosus_ under the most favorable conditions. The particle moved 3.56 times as fast as the ameba. Length of the ameba, 130 microns.]