Ameboid movement

by Asa A. Schaeffer

The mouth has considerable grasping power. This is shown in [Figure 32] where a filament of Oscillatoria was bent upon itself by the mouth and then rolled up in the body by the endoplasm in the same manner as a single filament. The mere viscosity of the endoplasm would be insufficient to bring about the bending of the filament. For the sake of comparison it should be added that a similar grasping power is also present in paramecium. The moment the food vacuole at the mouth is large enough, the endoplasm pulls it away and moves it rapidly toward the posterior end of the paramecium, much more rapidly than it would be carried by the rotationally streaming endoplasm. But from the posterior end forward the food vacuole is carried at the same rate as are the other particles in the endoplasm. In both Frontonia and paramecium rapid endoplasmic streaming precedes for a short distance the forward end of the ingested filament or the food vacuole ([Figure 32], a).

Figure 32. Showing ingestion of alga filaments in Frontonia leucas. a, the beginning of the ingestion of an alga filament. Note the streaming of the endoplasm preceding the end of the filament. b, almost two complete coils of the filament have been rolled up inside the Frontonia by the rotary streaming endoplasm. The endoplasm in the center of the animal is stationary. c, a filament, if thin, may be grasped anywhere along its length, bent together and swallowed in the usual manner. Diameter of a, 250 microns.

If a filament of alga is too long for the Frontonia, or one end of it is fast, streaming is reversed after several coils have been rolled up and the filament is ejected. So far as could be observed, the streaming process is reversed in all details, though the rate of ejection seemed to be somewhat slower than the rate of ingestion. Occasionally, however, ejection is accomplished much more quickly. If there are several coils of a filament whose other end is fast, rolled up inside of a Frontonia, the mouth sometimes stretches antero-posteriorly until the coil as a whole without unwinding is thrown out of the body. The viscosity of the endoplasm might lead one to expect that some of the endoplasm would be brought out with the alga, but such is not the case.

The essential differences between rotational streaming in Frontonia and in paramecium are: (1) It is under the control of the organism in Frontonia while in paramecium it is a continuous reversible process. (2) It is much more rapid in Frontonia than in paramecium. On the other hand, the physics of streaming in both organisms is essentially the same, so far as could be detected. In both organisms the energy of streaming is liberated within the endoplasm. This is especially well shown in the first stages of feeding.

Besides these organisms in which streaming occurs, either in a part of the organism or the whole, streaming is also found to occur in a great variety of plants other than those already mentioned; in the leukocytes of perhaps all coelomates; in some animal egg cells, such as the sponges, hydra and molluscs; in pigment cells, especially in batrachians and lacertilians; in phagocytes and wandering cells of a great many animals; in the nuclei of some animal cells; and in the intestinal epithelial cells of perhaps all metazoans. In almost none of these cases however do we know more than the bare fact that streaming occurs. No details are known. Consequently in so far as the purposes of this book are concerned it will not be apropos to discuss these cases further except to record the thesis that there is no evidence tending to show that these cases are not at bottom all characterized by the operation of the same fundamental process.

In all these cases of animal and plant cells and tissues in which ameboid movement occurs the process of streaming is easily observed in all of them, but the phenomenon of contractility is not noticeable in some cells except under special conditions, while in other cells it is operating continually. This indicates that there are other factors at work in addition to mere phase changes in the colloidal system to produce now contractility, now streaming. A high power of contractility and of streaming are not present in the same mass of protoplasm at the same time, though these powers may both be present at different times (Biomyxa).

Contractility can be explained in a general (though not yet in a detailed) way as due to a change in phase, more or less complete, in the colloidal system which is held to be the chief characteristic of the physical aspect of protoplasm. The change of phase is of course, associated with a change in the amount of surface energy, which is the ultimate source of the energy of contractility.

Streaming, however, does not depend upon a marked change of phase resulting in gelation, for observation has failed to detect this process going on to any extent whatever in streaming protoplasm. Further, an increase in the amount of water in the protoplasm is associated with more rapid streaming. If streaming therefore depends upon a phase change in a colloidal system, it must be in the direction of liquefaction, that is, changing the internal more fluid phase to the external phase. A phase change in one direction would thus lead to contractility, while a change in the other direction would lead to streaming.