The observations recorded in the preceding two chapters, while they do not tell us anything about the direct cause of the movement of the surface layer, nevertheless indicate clearly enough that the area where ectoplasm is made is the area toward which the surface film flows. There is no question therefore of the intimate relation between the transformation of endoplasm into ectoplasm and the movement of the surface layer.
The apparent absence of movement in the surface film of the pseudopods of Difflugia (Schaeffer, ’17) and the definitely proved absence of movement in the surface layer in the foraminiferan Biomyxa and myxomycete plasmodia, where no ectoplasm is formed in the manner observed in amebas, also indicates a causal relation between movement of the surface layer and ectoplasm formation. The relation moreover seems to be a necessary one for the movement of the surface layer is contrary to the processes involved in locomotion. In other words, from the standpoint of the ameba, it is a “necessary evil,” so far as locomotion is concerned.
The transformation of endoplasm into ectoplasm is unfortunately not understood, though from what we know in a general way of the behavior of colloidal solutions it seems to be a surface tension effect due to (or accompanying) a change of phase. Something akin to gelation occurs as Kite (’13) has shown. It is a problem in the chemistry of colloids. But the structure or composition of the protoplasm is complex and practically unknown, and it is quite open to criticism whether analogies from the behavior of pure solutions of colloids, such as gelatin, afford any real basis for an explanation.
Although a knowledge of the movements of the surface layer is interesting enough by itself, it will achieve its true importance only when it can be related to other processes in the ameba in a causal manner. It does not at present give us any greater insight into the ultimate cause of ameboid movement, although it is clear that an important step in this direction has been taken. But no theory of ameboid movement can be accepted that demands conditions in the ameba that are contrary to those described in the preceding chapters, in connection with the surface layer. From this point of view therefore the discovery of the true nature of the outside surface of the ameba is of importance, for it widens to a very considerable extent the observational basis with which any theory of ameboid movement must conform. Since the properties of the outer layer are here described in detail for the first time, it becomes necessary to enquire to what extent the more commonly held theories of ameboid movement conform with the observed behavior of the surface film. Although the surface tension theory was the first detailed theory proposed toward an explanation of ameboid movement, I shall discuss Jennings’ (’04) observations on the movements of the ameba first, because a great part of his work deals with the movements of the surface film, although he did not recognize it as distinct from the ectoplasm in its movements.
It is generally recognized that Dellinger’s (’06) work proved that Jennings’ conception of the ectoplasm as a permanent skin in which the ameba rolls along, is probably inadequate for such amebas as proteus; though singularly enough it is still supposed that verrucosa and its congeners move in the way described by Jennings (Hyman, ’17, p. 83).
Jennings (’04) describes the movements of amebas, both proteus and verrucosa “types,” as follows:
“In an advancing Amoeba substance flows forward on the upper surface, rolls over at the anterior edge, coming into contact with the substratum, then remaining quiet until the body of the ameba has passed over it. It then moves upward at the posterior end, and forward again on the upper surface, continuing in rotation as long as the ameba continues to progress. The motion of the upper surface is congruent with that of the endosarc, the two forming a single stream (p. 148).
“We have demonstrated above, for Amoeba at least, that the forward movement is not confined to a thin outer layer, but extends from the outer surface to the endosarc; in other words that the outer surface moves in continuity with the internal substance (p. 150).
“There is no regular transformation of endosarc into ectosarc at the anterior end. On the contrary the ectosarc here retains its continuity unbroken, moving across the anterior end in the same manner as across other parts of the body. In the same way the ectosarc is not regularly transformed into endosarc in the hinder part of the body.... Such transformation is by no means a regular accompaniment of locomotion” (p. 174).
According to Jennings, locomotion is aided by the projection of waves of hyaloplasm at the anterior edge, “an active movement of the protoplasm of a sort which has not been physically explained.” These waves, attaching themselves to the substratum, enable the ameba to pull itself along by a rolling movement as described in the quotation above.