From these observations on the formation of ridges it is evident that they do not represent a wrinkling of the surface such as occurs in a semi-rigid curved surface when it is made to occupy a smaller space. The ridges are wrinkles only in appearance, not in origin. The surface of the ridges is younger than the space between them. It appears as if the pseudopod which has to widen as it increases in length, could not liquify the ectoplasm uniformly all around, but only in longitudinal strips here and there, and that through these openings the ectoplasm then flows. There is no question about the greater readiness with which ectoplasm is formed in this ameba as compared with many others, but after a careful comparison of proteus and carolinensis, where ridges are formed, with discoides ([Figure 11], B), dubia ([Figure 11], C), laureata ([Figure 4]) and annulata, where none are formed, the only conclusion presenting itself is that the visible physical properties of the protoplasm of proteus and carolinensis give no hint as to the cause of the presence of ridges in these species. The protoplasm of discoides and laureata is about as viscous as that of proteus, yet in these there is never any ridge formation.
The ridges in proteus recall, of course, the ridges always observed in verrucosa, sphaeronucleosus ([Figure 13]) and their congeners, especially while the latter are in locomotion. A sphaeronucleosus is especially favorable for study in this connection because of its greater activity. This ameba has four or more longitudinal ridges on its upper surface, while in locomotion, which strongly resemble those in proteus and carolinensis. The chief difference lies in the fact that in sphaeronucleosus the ridges are extended at their anterior ends continually, and unless the direction of locomotion is changed, the ridges may retain their identity while the ameba moves several scores of times the length of its body. Along the sides, however, new ridges are continually replacing older ones. When the direction of locomotion is changed, the old ridges usually all disappear into a jumble of ridges and crinkles running in every conceivable direction, and with the reestablishment of locomotion along a more or less straight path, a new set of ridges appears. In sphaeronucleosus and its congeners, the ridges are also not wrinkles, but ridges that are formed later than the surface contiguous to them.
It is interesting to recall also that the ectoplasm in sphaeronucleosus, verrucosa and the rest of this group, is much firmer than in most other amebas.
CHAPTER V
Pseudopods and the Nature of the Ectoplasm
In contrast with the ridge-forming amebas stand those with smooth ectoplasm, such as the common dubia, discoides, villosa, and the rarer laureata and annulata, to mention only a few of the larger forms. In addition to these may be mentioned all the pelomyxas and nearly all the smaller amebas. Much the larger number of species of amebas do not form ridges in the ectoplasm during locomotion.
Figure 4. Amoeba laureata. This ameba is multinucleate, containing a thousand or more nuclei of the shape shown at the right. Ameba 1000 microns long in locomotion. Nuclei 10 microns in diameter.
Of all the amebas with smooth surfaces, the most favorable for observation as to the formation of ectoplasm, is the giant laureata ([Figure 4]), though it is unfortunately of infrequent occurrence. This species is as often found in clavate form as with pseudopods. In cross section it is circular or nearly so. It is often found with zoochlorella growing in it, upon which it seems to depend largely for food, for it seldom has distinctive food masses in it. The nuclei are small and very numerous and the crystals are well formed and numerous, each in a small vacuole, and of a size about two or three times those found in proteus. It will be seen therefore that there are only small bodies in this ameba, none of which (excepting the contractile vacuole) are large enough to change the course of the endoplasmic stream, and streaming is thus reduced to what might be called a typical condition.
In this ameba the endoplasmic stream flows uniformly towards the anterior end where it spreads out slightly so as to preserve the same general diameter of the ameba, for it is a characteristic of this ameba that the anterior end is of about the same diameter as the posterior, when in clavate form. The ectoplasmic tube is built at the anterior end, and remains as constructed until it is drawn in at the posterior end to form endoplasm. It is not all the time undergoing changes such as are observed in proteus. This characteristic is very well shown by focusing with the high power of the microscope on the upper surface of the ameba. The immobility of the ectoplasm is much more readily observed in laureata than in perhaps any other species, a condition that is due chiefly to the large crystals whose displacement is the most convenient criterion of ectoplasmic mobility.