Ameboid movement

by Asa A. Schaeffer

To say that the particles carried by the surface layer bring up at the anterior ends of pseudopods or of the ameba when in clavate shape, admits of further qualification. The advancing edge is not a straight line but an arc, and the sides near the advancing edge are building at a slower rate than the extreme tip. The most rapid formation of ectoplasm is at that point of the ameba that is farthest ahead. At this point all the ectoplasm to be made is still to be made, but as one passes back along the side of the pseudopod more and more ectoplasm is encountered and less and less remains to be made. There is therefore a gradient in the rate and in the amount of ectoplasm formed as one passes back from the forward end of the longitudinal axis of the pseudopod along the side. This is especially the case with certain amebas like Amoeba discoides, A. laureata and others in which the pseudopods are more nearly cylindrical. In such amebas as A. proteus and A. verrucosa, the factor of ridge formation complicates to some extent the longitudinal gradient of ectoplasm formation. But in spite of these specific differences, the general statement still holds that the rate of ectoplasm formation at the extreme anterior end is higher than anywhere else in the ameba, and that the rate gradually falls to zero as the nearly straight and parallel sides of the pseudopod or ameba, as the case may be, are approached.

Now we have seen that if a particle becomes attached to the outer layer of such an ameba as discoides, which has nearly symmetrical pseudopods, at some considerable distance from the tip of the pseudopod, it moves forward until the tip of the pseudopod is reached. It does not tend to come to rest near the tip of the pseudopod, where the rate of ectoplasm formation is much higher than at the sides of the pseudopod, though not as high as at the tip, but it moves on until the tip is reached. That is, the movement of particles on the surface film is toward that small area at the extreme anterior end where the rate of ectoplasm formation is highest.

In such an ameba as verrucosa, however, the highest rate of ectoplasm formation would be, not at a small circular area, but a very narrow strip along the anterior edge; for the rate of ectoplasm formation over a considerable portion of the width of the anterior end of the ameba is practically the same, according to observation. Consequently we do not find particles which are attached to the outer layer tending to move to a point lying on the longitudinal axis, but their paths are found to be straight and parallel with the longitudinal axis, if headed toward any point over a considerable stretch of the anterior edge on either side of the longitudinal axis.

All the evidence that is at hand therefore points to the conclusion that the direction of movement of the surface film in a moving ameba is toward that point where ectoplasm is formed most rapidly.

But where do the particles come from? At exactly what regions of the ameba do they start to travel toward the anterior ends of the ameba? In sphaeronucleosus and its congeners, it is very difficult to determine just when the particles begin to move toward the forward edge. Particles near the posterior end on the upper surface of these amebas moved forward slowly, much more slowly than particles near the middle. Sometimes particles near the posterior end seem to be motionless for some time, but the incessant though slow kneading process going on at the posterior end makes accurate observation difficult. Only in a general way it may be stated that particles begin their forward march at or near the posterior end. In amebas that habitually form pseudopods more accurate information can be obtained.

In proteus or discoides, for example, projecting pseudopods are often suddenly stopped and retracted, with a resultant change of an anterior to a posterior end. Particles attached to the outer surface on such pseudopods move toward the anterior end, of course, as long as the pseudopod is building, in the manner described in the preceding pages. But when the endoplasmic stream is arrested, the forward movement of the particle likewise stops. When the endoplasm starts to flow back into the main body of the ameba, the particle also starts moving back; but there is a period of a few seconds after the endoplasmic stream is reversed during which the particle remains quiet. And when it does start in to move, it moves only slowly. Within a few seconds, however, the average speed of movement is attained. This is true of particles located some distance away from the tip of the pseudopod. If the particle has reached the tip of the pseudopod before reversal of the endoplasmic stream takes place, the particle often remains at the tip until the pseudopod is almost completely withdrawn into the main body of the ameba ([Figure 26], p. 60). At other times such a particle becomes displaced, presumably by irregular retraction of the tip of the pseudopod, and finds itself at the side of the pseudopod. When this happens it moves slowly toward the main body of the ameba, but faster than the tip of the pseudopod does.

It frequently happens, especially in annulata, but also in proteus and other forms with many pseudopods, that when an advancing pseudopod is about to be withdrawn, there intervenes a stage where the endoplasm in the distal part moves away from the ameba, while that in the proximal part moves toward the ameba, with a neutral or motionless zone between. In such case a particle on the distal end moves slowly toward the tip while a particle in the proximal region moves toward the base of the pseudopod. Particles over the neutral zone are motionless. In these cases, however, changes in the direction and speed of the ectoplasmic stream are too frequent and the relative strengths of the distal and proximal currents too variable, to enable one to secure very accurate data by means of camera lucida drawings (a kinematograph is essential for this purpose), so no figures of the speed of movement of such particles are given. Nevertheless the general results of the observations are as stated. It might be added that in some cases the neutral zone for the particles attached to the surface did not coincide exactly with the neutral zone of the endoplasm, but was located a little further distally.

From these observations it appears that a rough index of the direction of movement of the surface film is the direction of the streaming of the endoplasm; and that the surface layer moves away from regions where ectoplasm is in the process of being converted into endoplasm. Since a particle attached to the surface may remain for some time at the tip of a retracting pseudopod, while one that is attached to the sides of a pseudopod moves toward its base, it appears that the speed of the moving surface film is not directly correlated to the rate of transformation of ectoplasm into endoplasm. The slower speed of particles near the posterior end points also in this direction. The formation of ectoplasm at the anterior end seems therefore to be much more intimately connected with the movement of the surface film than the destruction of the ectoplasm, though it is not yet clear that the liquefaction of the ectoplasm is altogether without effect.

Now as to the speed with which the surface film moves. The foregoing illustrations and figures show that the particles attached to a sphaeronucleosus on the upper surface move from 2.5 to 3.6 times as fast as the ameba ([Figure 19]) while particles attached to a discoides move only from 1.2 to 2 times as fast as the ameba moves. In proteus the speed of the particles is still slower, because of the longitudinal ridge-like waves of protoplasm which are continually being thrown out. In this species it frequently happens that because of the numerous ridges, the ameba moves faster than the particles attached to the outer surface; but this is to be looked upon as a mechanical complication, not as indicating a difference in the nature of the surface layer.

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.