It is well known that radiosa has two stages: a more or less clavate shaped stage in which the ameba creeps along the surface of some object ([Figure 7], d); and a stage in which a number (eight or less) of long and very slender tapering pseudopods are formed which usually persist for a long time ([Figure 7], a, b). These pseudopods are frequently quite straight and regularly disposed around the central mass of protoplasm (Penard, ’02, pp. 87, 89). In no case are any endoplasmic granules found in these slender pseudopods; they consist entirely of hyaloplasm. In retracting these pseudopods a curious phenomenon is sometimes observed; the pseudopod is rolled up into several (as many as six) turns of an almost perfect helical spiral of a diameter six to eight times that of the pseudopod. But as the process of withdrawal proceeds, the spiral becomes irregular, but parts of some of the turns persist in the last vestiges preceding complete withdrawal ([Figure 7], b). These spirals are also observed in other species besides radiosa (see p. 128 seq.)
Another species of ameba in which a trophic as well as a rayed stage is found, is the recently described species bigemma. In this species the rayed stage is only of occasional occurrence ([Figure 8], b). The larger the ameba is, the rarer is the rayed stage assumed. On very rare occasions one finds a rayed stage in which the pseudopods are long, straight, slender and tapering, and more or less regularly disposed around the central mass of protoplasm. The trophic stage ([Figure 8], a) is much the more common. In this condition pseudopods are formed in large number. They are small, conical or linear, and blunt, and they do not determine the direction of locomotion, as they do in proteus, dubia, or laureata. These pseudopods are often composed only of hyaloplasm, though frequently the basal parts of them consist of endoplasm. When these amebas become suspended in the water, they frequently assume a shape that approaches the rayed condition: six or more long conical pseudopods are run out from the central mass of protoplasm, but the pseudopods are not straight in this case, but irregularly curved and capable of being waved about to a slight extent. The ameba readily passes from this stage to the trophic.
The species Amoeba bilzi ([Figure 9]) has come under my observation on several occasions, and its pseudopodial characters are of considerable interest in this connection. In its usual form this ameba has the general appearance of a sphaeronucleosus.
Figure 9. Amoeba bilzi. a, the ameba in locomotion, showing the ectoplasmic ridges, nucleus, contractile vacuole. b, the transition stage between the rayed stage (which resembles that of radiosa, [Figure 5], p. 30, somewhat) and the stage shown at a. The whole of the ameba flows into the broad pseudopod with the arrow. Length of a, 90 microns.
In size it is about midway between the latter species and striata. It always has a number of prominent longitudinal ridges on its upper surface. Its mode of streaming is essentially like that of striata or sphaeronucleosus. When this ameba is disturbed and left suspended in the water, it throws out four or five or more long slender pseudopods composed entirely of hyaloplasm, excepting a bulbous base which consists of granular endoplasm. The pseudopods are cylindrical with tapering ends. They are very rigid, and once formed, persist for a considerable length of time. When these pseudopods are about to be retracted, the wall weakens at some point and then crinkles while the distal part of the pseudopod bends, often at a decided angle. The crinkling of the wall continues up and down the pseudopod while it is slowly being withdrawn. These pseudopods, as well as those of the rayed state in radiosa and bigemma, are not pseudopods of locomotion but of position; they are not dynamic but static structures. But there are no hard and fast distinctions to be made between these two types of pseudopods, for at least in bigemma and bilzi, there are transitional forms of pseudopods ([Figure 8], b).
The formation of pseudopods and their character depends to some extent upon the firmness and thickness of the ectoplasmic layer; and the character of the ectoplasm in turn depends largely upon the consistency of the protoplasm as a whole. In the following representative list of amebas: limicola, villosa, dubia, proteus, discoides, laureata, bigemma, bilzi, radiosa, sphaeronucleosus, verrucosa, the given order indicates a progressively thicker and firmer ectoplasm as one passes from limicola to verrucosa. But from limicola to bilzi the number of pseudopods directing locomotion increases from one to an average of about twelve in dubia, and then falls gradually to one in bilzi and the others beyond in the list. (See [Figure 10.]) Where the directive pseudopods begin to disappear, the transitional appear, viz., in bigemma and bilzi; but beyond these no transitional pseudopods occur. But along with the transitional there begin to appear also the static pseudopods, which are seen relatively seldom in bigemma and bilzi while in radiosa they occur at almost all times. In sphaeronucleosus and verrucosa no distinctive pseudopods of any kind occur.
If all the known species of amebas in which the necessary characteristics have been recorded, were arranged similarly with respect to the firmness and the thickness of the ectoplasm, the general relations of the various kinds of pseudopods in the list would be approximately the same as in the list given above; but there would appear an exception here and there, indicating the operation of special factors. Such an exception, for example, is seen in proteus in the list of species given, which because of the ridges that it forms (Figure 3) has a smaller number of pseudopods than would be the case if no ridges were formed[2]. It may be concluded, then, that the number and character of pseudopods depends in large part upon the ectoplasm-forming capacity of the ameba; and that this property is intimately associated with the degree of fluidity of the whole mass of protoplasm in the ameba.