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 But it is plain that as many pseudopods are withdrawn as are thrown out, and they are withdrawn at an angle against the main stream of endoplasm in the ameba that is the complement of the angle at which they were projected.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
Figure 2. Endoplasmic streaming involved in the formation of a typical food cup. a, the ameba is shown moving toward a live food organism that is resting quietly on the bottom. b, the main pseudopod forks, being the first indication that the feeding process has set in. At c the pseudopods have half-way surrounded the prey, but without having come into contact with it. At d the upper sheet of protoplasm, f, (stippled), is flowing dome-like over the prey, while the pseudopods continue to surround it. At e the pseudopods have met and fused with each other and the upper sheet of protoplasm has completely covered the space encircled by the pseudopods, and has fused with the pseudopods. g, sheets of protoplasm which are thrown out along the lower surface under the prey, to form a floor to the food cup. Up to stage e the ameba has not come into physical contact with the prey, but is just about to do so. With the completion of the floor of the food cup, the process of feeding is completed.
of the ameba when the ameba was carefully stimulated by means of very fine clean glass needles. The conclusion is unavoidable therefore that the shape of the food cup and the method of its formation is a racial characteristic and is hereditary. The streaming endoplasm therefore, upon suitable stimulation, takes on a definite form, that of a food cup. This indicates again that the endoplasm is something more than the ordinary fluids of physics, for out of an apparently structureless fluid, organization is effected.
The fact that food cups are formed by amebas implies of course that stimuli are received whose effect cannot be explained as a direct physical reaction. Rhumbler (’10) has attempted to explain the formation of food cups as the direct physical result of the stimulation by the food body; but in recent experiments Schaeffer (’16) has shown that food cups are formed over diffusing solutions of tyrosin, where the solutions were quite as concentrated outside as inside the cup. These results prove convincingly that the shape and size of the food cup are not determined by direct action of the stimulating agent, but by hereditary factors within the protoplasm of the ameba.
Other stimuli also affect streaming characteristically, though not so strikingly perhaps as food stimuli. One of the most widely observed effects on streaming is the momentary pause following stimulation of many sorts. If an ameba that is moving along unstimulated externally, suddenly comes near a food object, it frequently stops forward streaming for about a second, and then begins again, usually at increased speed. The ameba behaves as if it were startled. A similar reaction is observed if a small perpendicular beam of light is flashed near the anterior end of the ameba. Here also streaming is resumed with accelerated speed toward the beam of light. Harrington and Leaming (’00) showed that if strong light, especially at the blue end of the spectrum, is suddenly thrown on the ameba, movement is arrested for a short time. Miss Hyman (’17) has shown recently that if an ameba is strongly stimulated with a glass needle, streaming is arrested momentarily, but the direction of streaming when resumed subsequently, depends partly upon the former direction of streaming and partly upon the location of the stimulus. All of these cases of temporarily arrested movement are strikingly similar to what is observed in the higher animals under similar conditions.
The ingestion of a large food mass produces usually a marked change in streaming. A more or less spherical form is assumed, and if the food mass be a live organism such as a large ciliate, the ameba frequently remains quiet for a considerable interval. If a large amount of food is eaten, as for example a dozen or two colpidia, the ameba may suspend concerted streaming for an hour or more. During this time small pseudopods are projected here and there, but there is no locomotion. But if an ameba eats large masses of carmine, there is usually no pause following ingestion, and the same thing is true when the ameba is induced to eat bits of glass and other indigestible substances. It follows therefore that the interrupted streaming of the endoplasm due to feeding is not caused by the act of ingestion as such, but rather by the onset and continuance of the normal digestive processes on a large scale. These reactions are again strikingly similar to what is observed in many vertebrates, in which a more or less definite body sense, whose sense organs are in the splanchnic region, is supposed to be involved; but what the explanation of similar behavior in ameba is, is not at all clear.