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For the purpose of presenting in brief compass the main published observations and experiments on ameboid movement, we may pass from the observations of v. Rosenhof, mentioned in the introduction, to certain observations which Wallich (’63) recorded. He found that a new pseudopod is usually formed as a small break in the ectoplasm somewhere on the ameba through which the endoplasm then flows. As the endoplasm flows out and the new pseudopod enlarges, the breach in the ectoplasm increases in extent, through a transformation of the ectoplasm in the immediate vicinity of the breach, into endoplasm. But he observed also that some of the endoplasm which flows into the new pseudopod becomes transformed into ectoplasm. Wallich thus demonstrated that ectoplasm and endoplasm are mutually convertible.

The conversion of ectoplasm into endoplasm and vice versa, was regarded by Wallich, however, as a process taking place only occasionally, such as when new pseudopods are formed. It remained for Bütschli (’80, p. 115) to point out that in a moving ameba endoplasm is continually formed from ectoplasm at the anterior ends of all pseudopods, while the reverse process, viz., the conversion of ectoplasm into endoplasm, takes place continually at the posterior end of the ameba. He describes the relation of ectoplasm to endoplasm as a “circulation”; the endoplasm, arriving at the anterior end, becomes changed into ectoplasm, which after remaining relatively stationary for a while on the outer side of the animal, soon finds itself at the posterior end of the ameba, where it is slowly changed into endoplasm. The movement of the endoplasm forward to the anterior end of the ameba completes the cycle.

In 1898 Rhumbler, from observations on several species of amebas, came to the conclusion that in the change from ectoplasm into endoplasm, and vice versa, must be sought the cause of ameboid movement.

Jennings (’04), however, from extended study of the physiology of the ameba, stressing especially movement and feeding, denied that the transformation of endoplasm into ectoplasm, and vice versa, is necessary or even of frequent occurrence during movement. Instead of these transformations occurring regularly, as Bütschli and Rhumbler described them, Jennings concluded that the ectoplasm is more or less permanent, behaving like an elastic skin, which rolls over and over as the ameba moves along. The ectoplasm thus remains ectoplasm, and the endoplasm retains its identity, for considerable periods of time, instead of being continually transformed, the one into the other, as the ameba moves along.

Although observations with regard to movement in ameba have consisted almost wholly of the mutual relations of ectoplasm and endoplasm, it is important to note that the existence of a third layer of protoplasm, outside of the ectoplasm, was foreshadowed by an observation of Bütschli (’92, p. 219) while examining a pelomyxa. To his great surprise he found that there were currents of water, as evidenced by the movement of suspended particles, at the sides and in close contact with the ectoplasm of the pelomyxa, which flowed slowly forwards toward the anterior end. No details were given and no explanation offered for the cause of the currents excepting the suggestion that there might be a thin skin over the animal, which moves slowly forward.

Two years later Blochmann (’94) demonstrated by means of the very fine cilia-like projections which frequently cover the outside of pelomyxas, that the surface of the pelomyxa actually moves forward during active locomotion. He did not state definitely whether or not he considered this surface as a part of the ectoplasm.

This observation of Blochmann was not developed, however, until Jennings (’04), by means of particles attached to the outer surface of amebas, studied the forward movement of this layer. The results of Jennings’ work led him to conclude that the outer surface of amebas, which move forward as demonstrated by attached particles of soot and other substances, is continuous with the ectoplasm, and is really the ectoplasm. The rate of movement of this layer was stated to be about the same as that of the ameba as a whole. He denied the validity of Bütschli’s suggestion that there might be a thin third layer on the outside of amebas or pelomyxas.

But the existence of a third layer of protoplasm as distinct from the ectoplasm, was again maintained by Schaeffer (’17) who found that in some amebas the outer surface moves forward faster than the ameba advances through the water. The third layer was found to be generated over the surface of the ameba, especially in the posterior region of the ameba, and destroyed at the anterior end.

But the purely observational aspect of the problem of ameboid movement has not interested biologists generally as much as the ultimate cause of the phenomenon.

The first attempt that was made to explain ameboid movement in conformity with the demands of modern experimental science, that is, on the basis of physical factors, was made by Berthold (’86). By means of simple experiments with inert fluids (oils, alcohol, water, ether) which were modeled after an experiment described by the physicist Paalzow (’58), Berthold concluded that locomotion in ameboid organisms is due to the physical attraction of the anterior end to the substratum. The ameba was supposed to behave like a drop of fluid which moved towards the point where the tension of the ameba’s surface was decreased by contact with the substratum. The ameba did not push out pseudopods according to Berthold, but they were pulled out because of a difference in surface tension between them and the substratum. But pseudopods which were extended into the water and out of contact with a solid substratum, were said to be extended by a contractile effort of the posterior region of the ameba.

Bütschli (’92, p. 187) pointed out that it was highly improbable that pseudopods in contact with a solid substratum were projected in a fundamentally different way from that in which free pseudopods were extended, as explained by Berthold. Bütschli assumed that all ameboid movement was due to the same fundamental cause. He postulated surface tension as the active agent, as Berthold had done for the extension of pseudopods in contact with a solid substrate; but Bütschli assumed that the decrease in surface tension at the anterior end of the ameba was brought about by the bursting of protoplasmic droplets of a more fluid consistency on the surface of the ameba, the consistency of which was less fluid, thus bringing about a decrease of surface tension and consequent forward streaming of the endoplasm. The necessary migration of the more fluid droplets to the surface was determined by internal conditions. The direction in which an ameba moves was assumed to depend therefore not upon the physical character of the substrate, as suggested by Berthold, but upon such internal changes as control the movement of the more liquid part of the internal protoplasm to the outer surface.

Rhumbler (’98) wrote extensively on the subject of ameboid movement, especially from the point of view of the feeding habits of amebas. He concluded that the flow of protoplasm, while engulfing a food object, was a direct result of the lowering of the surface tension of the protoplasm by contact with the food object (p. 207), thus causing its envelopment. Numerous other writers of the time, including Quincke (’88), Verworn (’89, ’92), Blochmann (’94), Bernstein (’00) and Jensen (’02), agreed in a general way with Rhumbler’s position that surface tension changes are the cause of locomotion in ameba.

In 1904 the general subject of ameban behavior was extensively studied by Jennings, and from his observations he concluded that surface tension cannot account for many of the reactions observed. Other factors, he held, must be at work, such as contractility, which, acting in the posterior region, causes the endoplasm to flow forward. But Jennings found it impossible to explain on the same basis the extension of free pseudopods, and the creeping of a pseudopod, or of the whole ameba, over a solid substratum.

From further observations Rhumbler (’05, ’10) came to modify his earlier views as stated above. The rapid advances in the study of the chemistry of colloids doubtless suggested to Rhumbler that the change from endoplasm to ectoplasm resembled the change from a sol to a gel state, and that in this process of gelation lay the source of energy manifested in ameboid movement. In thus calling attention to, and emphasizing the colloidal nature of, the conversion of endoplasm into ectoplasm and vice versa, the problem of ameboid movement came to be discussed from an entirely new angle. Certain phases of Rhumbler’s theory are developed and elaborated by Hyman (’17) who agrees in general with Rhumbler’s conclusions.

In a series of papers on feeding and other reactions of ameba, Schaeffer (’12, ’16, ’17) concluded that Rhumbler’s general statement, wherein he says that changes in behavior are directly deducible from the action of stimuli in effecting liquefaction or gelation of the ectoplasm, does not hold in many cases of feeding, and that the mechanism controlling locomotion and feeding is not external, as maintained by Rhumbler, but internal.