One more point needs mention in this connection, and that is the small waves of clear protoplasm which are thrown out by many amebas at their anterior ends during locomotion. They are especially prominent in A. bigemma ([Figure 7]) and in radiosa ([Figure 8]), but they are formed in perhaps all species. Observation does not indicate that they move in exactly the same way as the main body of the endoplasm, even if the larger granules could be left out of account. They behave more like the clear pseudopods of Difflugia and Arcella and the foraminifera.
Although these waves are frequently not to be seen during locomotion in Amoeba proteus and other large amebas, particularly in Pelomyxa palustris and P. belevskii, it is possible that the wave forming process has become indistinguishably merged with endoplasmic streaming. It is not impossible that the projection of these waves is the purest expression of ameboid movement. But on account of their small size and transparency, it is very much more difficult to investigate them than streaming of the granular endoplasm, as it is observed in amebas, ciliates and plant cells. It seems to be true however that streaming can occur in the entire absence of these waves, so their importance in ameboid movement is probably secondary.
CHAPTER X
Streaming, Contractility and Ameboid Movement
The nearest relatives of the amebas are the shelled rhizopods, the Difflugias and the Arcellas and their congeners. The movement of these organisms is quite different from that of the amebas in that the whole body of the endoplasm does not stream into the pseudopods, but only a small portion of it. There is consequently no regular transformation of ectoplasm into endoplasm at the posterior end, that is, the protoplasmic mass within the shell. The method of movement in Difflugia was described by Dellinger (’06). A pseudopod is thrown out to a considerable distance. It fastens itself to the substrate at the tip. It then contracts, pulling the Difflugia forwards. While this pseudopod is contracting, another one is extended in the same direction. When it has arrived at the maximum length, it fastens itself at the tip and then contracts, pulling the Difflugia along. Continued locomotion consists of a repetition of this process. The pseudopods are slender and consist nearly always of clear protoplasm. Only occasionally does one see conspicuous endoplasmic granules flow into a pseudopod, and then only at the base.
The transparency of the pseudopods in Difflugia and the absence of granules in the protoplasm composing them, prevents one from seeing clearly how the pseudopods are formed, that is, whether or not there is a regular transformation of endoplasm into ectoplasm at the anterior end. The fact that one occasionally sees the endoplasm stream into the base of a pseudopod in the same way as was described for ameban pseudopods, indicates that the method of formation of pseudopods in Difflugia is in general similar to that in ameba. But the process is not exactly the same, for the surface layer on the pseudopods of Difflugia does not move as fast as the tips of the pseudopods advance, while in amebas the surface layer moves faster than the pseudopods. What this difference indicates has not yet been ascertained.
The protoplasm of the pseudopods of Difflugia is thick and the power of contractility highly developed, for the pseudopods readily move about in the water like a tentacle. The demarcation line between ectoplasm and endoplasm is very difficult to see, consequently no definite idea can be given as to the thickness of the ectoplasm. When a pseudopod is being extended the whole contents seem to move at about the same rate as the pseudopod advances, differing thus from amebas, in the pseudopods of which the central core of the endoplasmic stream flows considerably faster than the tip of the pseudopod advances through the water. But when a large pseudopod is cut off from a Difflugia it is able to move after the manner of an ameba without a nucleus (Verworn, ’94).
In heliozoans protoplasmic streaming is quite different from that in ameba or Difflugia. The pseudopods are usually straight, radiating from the central body. They possess usually a central axial rod of condensed or strongly gelatinized protoplasm around which is a layer of thick protoplasm with the properties of ectoplasm. Heliozoans for the most part move slowly; in fact many of them are pelagic and in these the power of locomotion on a solid substratum is very slow. There is however one species, Acanthocystis ludibunda, which, according to Penard (’04), can move twenty times its diameter in one minute by rolling. This illustrates a highly developed power of contractility in the pseudopods of this organism, for since only about one-fifth of the circumference can be in contact with the solid substratum, the pseudopods must attach themselves, contract so as to pull the Acanthocystis along, and relax their hold, all in the space of two seconds.
Among pseudopod forming organisms, the highest development of contractility is found in the foraminifera. As is well known, these organisms form finely anastomosing pseudopods which frequently cover the substratum with a network of protoplasmic strands. The terminal sections of these strands are frequently so thin and transparent that they cannot be seen easily with the microscope. As a rule the granular endoplasm is observable only in the main body of the organism and in the larger trunks of the pseudopods. Much the larger part of the pseudopods, as measured lineally, is devoid of granular endoplasm. The great power of contractility and the speed with which contraction may occur in Biomyxa, a fresh water foraminifer, have already been mentioned ([Figure 12], p. 47). Similar observations have been recorded by other observers, recently by Schultz (’15), who compares the contractility of foraminiferan pseudopods to that of rubber bands. In fact as one watches the movements of a Biomyxa, for example, under moderately high magnification, one gains the impression that there seems to be no restriction imposed upon the extent of contractility in the pseudopods. They seem to possess perfect elasticity. As to the transformation of endoplasm into ectoplasm, little can be said, owing to the transparency of the protoplasm. But the whole of the pseudopod, when forming, seems to stream forward. As in Difflugia, the interior streams flow at about the same rate as the pseudopod as a whole advances. The highly developed power of contractility however demands rapid changes in phase of the colloidal system, and also a thick consistency. The behavior of pieces of the pseudopodial network, when cut from a Biomyxa, shows clearly that the protoplasm is actually thick, as compared with that of an Amoeba proteus. When a Biomyxa is contracted into a spherical mass, the interior exhibits continual rapidly streaming movements. Some of these are rotational but most of them are radial. All of the streams frequently change their direction and extent. No corresponding changes are visible in the outer peripheral layer.
Among plants, some of the algae possess ameboid protoplasts at one stage or another of their life cycle, but the details of streaming have not been made out. It has been reported however that the zoospores of some parasitic fungi move to all appearances exactly like small amebas. We likewise lack details of the streaming of the myxomycete plasmodia. From a more or less cursory examination of a small aquatic plasmodium of undetermined species, it appeared that the formation of pseudopods and the process of streaming were quite different from similar processes in the foraminifera. The pseudopods do not act independently as in foraminifera. At almost the same moment the protoplasm begins to flow from the pseudopods in a large section of the plasmodium and into another section; then soon thereafter the protoplasm flows back again. This oscillatory streaming is continued presumably as long as the myxomycete is in the plasmodial stage. With every change in the direction of movement of streaming, there is produced, however, a change in the shapes of the pseudopods, so that with a number of oscillations in streaming an appreciable degree of locomotion is effected. The direction of locomotion can be markedly affected by changes in light intensity and moisture distribution, as shown by the observations of Baranetzsky (’76), Stahl (’84) and others, but just how these changes in the direction of locomotion were produced is not recorded. There is a definite ectoplasm and a definite endoplasm in the myxomycete plasmodia, but the details of their transformations, the one into the other, have not been determined; but since the surface layer is stationary, it is probable that there is no such regular transformation of endoplasm into ectoplasm at the anterior ends of pseudopods as there is in ameba. But this phase of the subject needs further investigation before any conclusions can be drawn. The power of contractility is present, but apparently only to a slight degree. Too little is known of the streaming process in these organisms to compare it in detail with the same phenomenon in rhizopods.
The streaming of protoplasm in plants has received a good deal of attention, though only comparatively little experimental work has been done. Streaming is observed in a great many plant cells, and in some cells such as the large internodal cells of Chara and Nitella, the process may be easily observed. The essential features of a plant cell in which streaming occurs are, first, the external cell wall of cellulose, which of course prevents any change of shape in the cell such as is observed in naked protoplasts as, for example, ameba. Inside of the cell wall is a layer of ectoplasm which has essentially the same properties as the ectoplasm of amebas. In some cells such as those of Chara, the ectoplasmic layer is thick and contains nearly all the chloroplastids, while in the leaf cells of Elodea the ectoplasm is extremely thin and is practically free from chloroplastids. In the interior of the cell are found the streaming endoplasm and one or more large vacuoles filled with cell sap.