Another interesting property of reticulose pseudopods, which are formed by a streaming process, is their great power in some species, of rapid contraction. If a diatom for example, in its movements breaks loose a pseudopod it is often (though not necessarily) contracted very rapidly, much more rapidly than could be the case if it were accomplished by streaming. It frequently happens that knobs are found on a slender pseudopod. These knobs may move back and forth with great rapidity without visibly affecting the pseudopod (Figure 12). The process reminds one of a block sliding on a rope. These observations indicate a very high degree of elasticity in the formed pseudopods of such a rhizopod as _Biomyxa_ as compared with a very low degree of elasticity in the amebas. 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. [Illustration: 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.