It is well known that in a number of the small amebas, especially the soil amebas, there are two trophic stages, an ameboid stage and a free swimming flagellate stage. The change from one stage to the other is a matter of a few minutes only. In the flagellate stage ([Figure 43]) the amebas resemble a small flagellate like chilomonas, very closely. Their manner of swimming is very similar. And it is especially noteworthy in this respect that they revolve on their long axis and describe a well marked, regular spiral path, just as do the flagellates and ciliates. Unfortunately no records have yet been made of the paths these amebas describe when in the true ameboid stage. Since, therefore, as we shall see later, the slightly unsymmetrical shape of the flagellate stage is not the cause of the spiral path, it is probable that the mechanism controlling the activity of the flagellum can produce orderly locomotion only when the organism follows a spiral path.
Much has been written about the fundamental similarity or identity between flagella and pseudopods. All writers who have expressed themselves on this point incline to think that there is such similarity, that flagella are really very slender and very agile pseudopods. I am not going to record here the evidence for this conclusion, for I have recently had the good fortune to make some very convincing observations on a hitherto undescribed ameba
Figure 43. The flagellate stage of a soil ameba, after Wilson. a, stained preparation showing the two flagella arising from the blepharoplast, d, which is connected with the caryosome, c, the central chromatin mass. Much of the chromatin is deposited on the nuclear membrane. b, a drawing from a live flagellate showing flagella, nucleus, c, and a vacuole.
(which for the sake of reference will here be called flagellipodia, [Figure 44]) whose pseudopodia stand about midway between typical flagella and typical pseudopods in their activity. In its general characteristics it stands near A. radiosa, but quite unlike the stiff, static pseudopods which radiosa very frequently forms, this ameba has usually five or more slender pseudopods of which one or two or more are in slow flagellate motion. The distal third or half of the pseudopod is in the shape of a corkscrew. The free end of the pseudopod travels around in a circle (anti-clockwise in all instances observed), making one revolution in about three seconds. If this motion were very rapid it would act like a propeller and the ameba would swim through the water. The part of the pseudopod back of the mobile portion is usually also thrown into a spiral of gradually decreasing diameter until the spirality disappears. This portion of the pseudopod is not mobile in the same way that the distal portion is. Sometimes the whole of a pseudopod is thrown into a spiral, all of the turns being of equal size and only slightly motile. More than half of all the pseudopods formed become spiralized at one time or another of their existence, the greater number of these being however relatively immotile. Pseudopods frequently fall into spirals while they are being extended.
Figure 44. Amoeba flagellipodia. a, showing nucleus, 4 microns in diameter, and four vacuoles. b, a pseudopod of three spiral turns which in a few seconds grew into one of six spiral turns, c. d, a pseudopod of a number of spiral turns, which a few seconds later took on a shape shown at e. The tip of the pseudopod at f turned screw-like anti-clockwise, when looking at the tip and at the main body of the ameba. The tip made one complete revolution in about three seconds.
A better transition form between pseudopods and such flagella as are found, for example, in the peranemas, could hardly be imagined. The difference between crawling and swimming would seem to be merely a matter of speed of movement of the pseudopod.[6] But important as such a transition form is for theoretical purposes in understanding the nature of both flagella and pseudopods, it is of special importance for our present purpose because it shows a strong tendency for pseudopods to fall into spirals and to move in spirals. This tendency is found not only in this species of ameba but is observed also occasionally in radiosa ([Figure 7], p. 30) and in several other species. In these latter species the pseudopods are stiff and not capable of waving about in the water, as are those of flagellipodia, whether in the spiral shape or not. In radiosa the pseudopods may become spiralized only as a preliminary to withdrawal. It is evident therefore that the spiral urge can express itself best in a plastic pseudopod.