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.
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.
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.
The movement of particles on the under side of an Amoeba sphaeronucleosus depends upon what part of the ameba is attached to the substratum. Where the ameba is attached there is of course no movement of the surface layer and the particles remain stationary. In an ameba attached as shown in figure 20, a, there was a very slow movement of particles forward near the middle of the attached region (x), but whether this was related to the movement of the outer layer of the upper surface was not determined. The movement of these particles was considerably slower than the movement of the ameba. In another ameba attached at the anterior and posterior ends ([Figure 20], b) no movement of particles on the under side could be discerned. The small particles showing Brownian movement, with the surrounding water, are dragged along as a mass. This movement is purely mechanical, and is what would be expected on purely physical grounds, when a more or less cup-shaped object is moved along in water in close contact with a flat surface. Such particles as have become attached to the surface layer on the under side of the ameba, because of their slower movement than that of the ameba, eventually bring up at the sides near the posterior end, as the ameba moves along. From here they are carried forward in the manner already described. Thus there comes about a “rotation” of particles adhering to an ameba as described by Jennings (’04) and Dellinger (’06), though the explanation is different from that given by Jennings (l. c.) as we shall see further on. No case of a similar rotation of larger particles which had sunk into the ectoplasm, as described by Jennings (’04, p. 142), has come under my observation.