It thus appears that the process of streaming is a much more fundamental phenomenon than most of the theories accounting for ameboid movement would lead one to suppose; for these theories concern themselves only with streaming as observed in amebas, and many content themselves with only two or three species. Since the general features of streaming are similar no matter where streaming occurs, no theory is likely to gain acceptance that explains streaming only in one group of organisms. Streaming in rhizopods, myxomycetes, ciliates, plant cells, is most rationally looked upon as caused by the same fundamental process; but the detailed form it takes, especially in freely formed pseudopods, is undoubtedly conditioned by the structure of the protoplasm, both physically and chemically, but more especially the latter.
Figure 12. Illustrating the high degree of elasticity in the pseudopods of Biomyxa vagans. In a and b are shown two stages of a small section of the pseudopodial network, which remained unchanged while a small lump of protoplasm (near the arrow) moved rapidly up and down the slender pseudopod. Movement along the whole length of the pseudopod occupied about half a second. Just exactly what the movement was due to could not be determined, but the distance between the forks in the pseudopod did not change, nor did the thickness of the protoplasmic strand on which the protoplasmic lump moved change noticeably.
CHAPTER VII
Experiments on the Surface Layer of the Ameba
In the preceding chapters we have discussed the streaming of the endoplasm in various representative species of ameba, and its transformation into ectoplasm at the anterior end. We have observed that the details of streaming are not quite the same for any two species of ameba, and that in consequence the character of locomotion also is specific for every ameba. All the observations prove that movement in ameba is always associated with streaming, and streaming (in locomotion) with ectoplasm formation. It follows therefore that the form of movement observed in amebas depends invariably upon the streaming of the endoplasm accompanied by the formation of ectoplasm.
There is however another element which, although it appears to be a consequence of ectoplasm formation, must nevertheless be included in any account of ameboid movement because of the light it is bound to shed on the physical processes concerned in streaming. This element is the thin outer layer which separates the water in which the ameba lives from the ectoplasm. It is the properties of this layer to which we may now direct our attention.
That such a layer exists was indicated by observations of Bütschli (’92) and Blochmann (’94), as already mentioned; but neither of these authors stated definitely whether they considered a third layer actually to exist or whether the ectoplasm as such moved forward. Jennings (’04), as has been pointed out, concluded that no third layer exists and that the particles clinging to the outsides of amebas, which are carried toward the anterior end, are carried by the ectoplasm. Gruber (’12) concluded however that an outer layer exists, composed of gelatinous substance, which moves ahead at about the same rate as the ectoplasm (p. 373). According to Gruber’s view the outer layer is a permanently differentiated layer of material. Schaeffer (’17), on the contrary calls it a layer of protoplasm, which moves forward faster than the forward advance of the ameba.
It is a very simple matter to demonstrate the existence of this layer. Although any insoluble non-toxic substance of low specific gravity such as carmine or soot, when reduced to very small particles and mixed with the water in which the amebas to be examined live, will cling to the outside of the ameba so that the movement of the outer layer can be observed; in my experience the best as well as the most convenient substance to use is the dried flocculent colloidal sediment from ameba cultures, rubbed to powder with the ball of the finger. This powder swells up in water into flocculent masses which are large for their weight and do not show such active Brownian movement as particles of carmine or india ink, and they consequently adhere more easily to the ameba. Moreover no foreign substances are thereby introduced into the water.