Figure 27. A part of an Amoeba proteus illustrating what is perhaps the most characteristic quality of the surface layer of amebas, its fluid nature. Three particles, a, b, c, were moving forward along an actively growing pseudopod. In stage 2, particles b and c had arrived nearly at the tip of the pseudopod. A pseudopod was then thrown out on the right, which resulted in the movement of a in the same direction, while b and c remained nearly stationary. Later on this pseudopod was retracted. b and c were drawn back toward the main body of the ameba while c remained behind, moving only very slowly. Thus the relative positions of these particles was completely changed.
by a line for convenience of reference, were in the position indicated at 1 when the forward end of the ameba occupied the position indicated by outline 1. As the ameba moved forward the particle c gained slightly on a and b for no ascertainable reason, unless it was on account of the projection of the large pseudopod on the opposite side. At stage 2 a new pseudopod was started on the right, which at stage 3 had grown to large size while streaming in the original pseudopod was arrested. At stage 3 particles a and b retained the same position they had in stage 2, except for a slight turning to the right. Particle c however moved across the base of the original pseudopod and on to the middle of the new pseudopod. At stage 4 a and b had again only slightly moved to the right of the position they occupied in stages 2 and 3, while c moved rapidly toward the tip of the new pseudopod. The new pseudopod was then retracted and at stage 5 the particles had begun to move back toward the main body of the ameba. Particles a and b now gained considerably on c because they were located further away from the tip of the retracting pseudopod. Particles a and b were drawn to the middle of the retracting pseudopod because of the continuous enlargement of the large pseudopod on the right, below, through which the ameba moved on.
The most important feature of this observation is the change in the position of the particle c with respect to that of a and b. The latter particles retained their relative positions with very slight, if any, change, while c swung around a and b nearly 180°, and at the same time changed the distance very greatly between itself and the other particles. Moreover, b, at stage 5 led the procession of particles, while at stage 1, a led. No further demonstration is necessary to show that the surface layer is distinctly fluid and dynamic, and not at all such a static structure as an elastic permanent skin, as Jennings (’04) and Rhumbler (’14) maintained.
CHAPTER VIII
ON THE NATURE OF THE SURFACE LAYER
The observations in the preceding chapters on the general movements of the surface layer of amebas will afford a sufficient basis for an inquiry into the nature of this layer. The mere demonstration of the existence of this layer is, of course, interesting enough, for a number of contradictory statements by various students of the amebas are satisfactorily cleared up by these observations. But the problem of ameboid movement affects other organisms besides amebas, and since the movement of the surface layer is so intimately associated with ameboid movement, it becomes of more than ordinary interest to learn something of the nature and composition of this layer.
In the first place the property of carrying particles toward the anterior end of amebas does not appear to be of any advantage. That is, whatever the movements of the outer layer may be, the ameba does not appear to be better off when particles are carried forward than when none are carried, for such particles are very small and almost without exception devoid of food value. The particles are masses of debris which accidentally adhere to the ameba, and the ameba makes no visible effort to make such particles adhere, nor to get rid of them. The ameba seems to be quite indifferent to the presence of such particles.
On the other hand, as Schaeffer (’17) has pointed out, the capacity for transporting particles cannot but be looked upon as a hindrance to locomotion. As has been stated, the surface film moves in the same direction as the ameba. Whenever the surface film comes against a solid object, it pushes against the object, and nullifies to a certain, though small, extent the energy expended in moving forward. And it will be seen without further argument, of course, that the energy involved in carrying particles forward is not only itself lost but consumes an appreciable part of the energy available for forward movement. This fact, together with the universal occurrence of this phenomenon among amebas indicates beyond question that it is intimately associated with ameboid movement as it is ordinarily understood in amebas, and that it is almost certainly a “necessary” physical consequence of the more fundamental physical processes involved in the movement of amebas.
That the third layer moves in the same general direction as the ameba has already been mentioned. The direction of a moving particle is however not necessarily parallel with the stream of endoplasm below. In a retracting pseudopod that lies nearly parallel to and by the side of the main advancing pseudopod, the particles on the far side and near the base frequently move across the pseudopod at an angle (and therefore also across the endoplasmic stream), and up the active pseudopod on the near side. This shows conclusively that the direction of flowing endoplasm by itself has no direct connection with the direction of flow of the surface layer.