Ameboid movement

by Asa A. Schaeffer

As to the rate of movement of the outer surface as compared with that of the endoplasm, Jennings concluded:

“The direction of movement of particles on the outer surface is the same as that of the underlying particles of endosarc. The rate is also about the same as for the endosarc, though often, or perhaps usually, the outer particles move a little more slowly than those in the endosarc” (p. 142).

In view of the observations recorded in the preceding pages it is clear that Jennings’ statement that substance after moving forward on the upper surface, rolls over the anterior edge is quite erroneous. The attached particles, if heavy, may do so, but the surface film itself does not. It is, on the contrary, taken into the interior at the anterior edge.

The statement that the movement of the outer surface is congruent with that of the ectoplasm can likewise not be substantiated by observation, as has been demonstrated in the preceding pages. It is difficult to distinguish between the ectoplasm and the surface layer in such amebas as sphaeronucleosus and verrucosa, for there are no large crystals or other bodies which get caught in the ectoplasm as it is formed from endoplasm at the anterior end. But attentive observation will demonstrate very definitely that the ectoplasm here is stationary to the same degree as in proteus. The stationary properties of the ectoplasm are however not properly a matter for discussion; for five minutes’ observation of a proteus, discoides, annulata, particularly a laureata, under 300 diameters magnification, will convince anyone that the ectoplasm is stationary while the surface film, with attached particles, moves over it. No one can possibly come to any other conclusion. Jennings’ conclusion was due undoubtedly to an error of observation.

Jennings’ statement that the rate of movement of the outer surface is the same as that of the endoplasm (p. 142) when taken in connection with his other statement that the ectoplasm is a more or less permanent skin, presents a mechanical impossibility; for unless the outer surface moves twice as fast as the endoplasm, no rolling movement would be possible. Several of Jennings’ figures (especially Figures 38, 39, and 41) indicate in fact that he conceived of the outer surface as moving faster than the ameba advances, or that the upper surface moves over the ameba as the ameba moves over the substrate. Jennings’ theory requires that the surface layer move twice as fast as the ameba advances. Hyman (’17) also makes a similar mistake in referring to the rate of movement of the outer surface (p. 85).

Lest the discussion of this point be suspected of being merely verbalistic, it should be recalled that the surface layer of proteus often moves at about the same rate as the ameba; that the surface layer of discoides moves about twice as fast as the ameba; that the surface layer of verrucosa and sphaeronucleosus moves about three times as fast as the ameba; and that the ectoplasm does not move at all. It is of course incumbent on one to discuss what is stated; one is not at liberty to select one of several possible interpretations.

To illustrate this point graphically so as to avoid as far as possible future confusion [Figure 29] is appended. In a is shown a particle traveling on an ameba at the same rate of speed as the ameba; at b is shown a particle that moves twice as fast as the ameba; at c the attached heavy particle does not move at all. For the sake of completeness d, [Figure 29], is added here. It illustrates the backward moving ectoplasm in an ameba that is suspended in a jelly medium that prevents the ameba from sinking to the bottom. It must be admitted that in thus considering the rate of movement of the various tissues of the ameba from a single standpoint, a point outside of the ameba, little room is left for confusion.

Figure 29. a, a particle attached to an ameba and moving at the same rate as the ameba. This condition is often observed in proteus where the surface film, owing to its destruction during the formation of the longitudinal ridges, retards the forward movement of this layer. b, a particle attached to the surface film of an ameba moving twice as fast as the ameba. This condition is seen in discoides, verrucosa, sphaeronucleosus, etc. c, a particle on an ameba that does not move at all although the ameba does. This is seen when a heavy particle is laid on an ameba, too heavy for the surface film to move. d, movement of ectoplasm in an ameba suspended in a jelly medium. The vertical lines are to be considered as stationary.