Ameboid movement. Asa A. Schaeffer

      Figure 1. Illustrating the various directions of endoplasmic streaming in growing and retracting pseudopods. a, two oppositely directed streams in a pseudopod, one directed toward the base and the other toward the tip of the pseudopod, with a neutral zone between. b, two streams flowing toward each other. Cases c to r are self explanatory. s, rotational currents observed occasionally in various species of amebas. t, “fountain currents,” sometimes observed in Amoeba blattae, and rarely in other forms. u and v represent cases of streaming which have not been observed and which probably do not occur. w, similar to v, but with a wide neutral zone between the streams, represents an actual observed case. m and r probably occur only very rarely; no such cases have been seen, but there seems to be no reason why they do not sometimes occur. Excepting m, u, r and v, all these figures were drawn from observed cases of streaming.

      ectoplasmic tube, in an ameba of several pseudopods, excepting where there is a wide zone of stationary endoplasm between the streams (Figure 1, v, w). But in “fountain currents,” such as Rhumbler (’98, p. 190) described and figured for Amoeba blattae Bütschli, and which may readily be observed in most species of amebas if immersed in a solution of gelatin thick enough to keep the amebas from sinking, there is a central stream of endoplasm flowing forward, and a peripheral stream of ectoplasm flowing backward, with a thin neutral zone between (Figure 29, d). As we shall see further on, however, these fountain currents are in principle the same as the currents observed in ordinary locomotion, the apparent difference being due to the fact that there is no locomotion. It is true, then, that within the same pseudopod at any cross section the endoplasm always streams in one direction, and the streaming is unified.

      When new pseudopods are formed, or when old ones are retracted, and especially when both these phenomena occur at the same time and close together on a part of an older pseudopod, some of the details of coordination in streaming are readily made out. In Figure 1 are shown a number of observed cases of pseudopod formation and retraction, with the direction of endoplasmic streams indicated at a given instant. For the purpose of illustration, several (presumably) possible but unobserved cases, m and r, are sketched, and also two cases, u and v, which have not been observed and which probably do not occur. The general conclusion to be drawn from these observations is that, while the endoplasm in the body of an ameba as a whole may be streaming in several different directions at any given instant, that is almost never the case with an individual pseudopod, especially if the pseudopod is of small or medium size and not too flat or otherwise irregular in shape. The pseudopod is therefore the unit of coordinated protoplasmic streaming.

      Another general observation which undoubtedly is connected in some way with the problem of coordinated streaming is the following. In externally unstimulated amebas, the new pseudopods are almost without exception directed 60° or less from the direction in which the parent pseudopods are moving.

      It is a matter of common observation that an ameba may throw out a pseudopod in any direction whatsoever when stimulated. The ameba may reverse its direction of movement completely, or it may move in scores of different directions at one time for awhile, if properly stimulated. There is no restraint or limit imposed upon the ameba insofar as the direction of movement is concerned. Why then should a great majority of new pseudopods in an unstimulated ameba be projected at an angle of approximately 60° to the parent pseudopod? It might seem at first sight as if the merely physical aspect of the streaming would be a sufficient explanation, in that less resistance would be met with in sending a stream off at a small angle than at a large. But it is probable that inertia plays no part in maintaining the direction of streaming (see p. 123, footnote, for further discussion). It requires perhaps more energy for a pseudopod to flow off from the main stream at an angle of 120° than at an angle of 30°. But it is plain that as many pseudopods are withdrawn as are thrown out, and they are withdrawn at an angle against the main stream of endoplasm in the ameba that is the complement of the angle at which they were projected. Whatever energy might be saved therefore in the projection of a new pseudopod at a small angle with the main stream is lost in withdrawing the pseudopod against the stream at a correspondingly large angle. It is clear therefore that the physics of moving viscous fluids cannot solve the problem. It is probable that the mechanism which controls the direction of locomotion as exemplified in the wavy path of the ameba (see p. 109) is also involved in the direction in which pseudopods are projected.

      Some very interesting special cases of endoplasmic streaming are observed during the process of feeding. As is well known, amebas capture their food by the protoplasm flowing around it and engulfing it. If the object is large the protoplasm may flow around it, in contact with it, so that the shape of the object determines the direction in which the enveloping protoplasm flows. If the object is small, particularly if it is a live organism, the behavior of the ameba is quite different (Kepner and Taliaferro, ’13, Schaeffer, ’16). To capture such a food object a cup of protoplasm is gradually formed over it so as to imprison it (Figure 2). If the food organism lies against some flat object, the food cup is brought down to the surface of the object all around, thus making escape impossible, before the protoplasm comes into contact with the food organism. Schaeffer (’16, ’18) by experimental methods has shown that the stimulus calling forth the formation of food cups as just described, is the mechanical vibration of the water. At least the same response was produced on the part