CHAPTER IX. The Surface Layer and Theories of Ameboid Movement

THE SURFACE LAYER AND THEORIES OF AMEBOID MOVEMENT

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.

As to the rate of movement of the outer surface as compared with that of the endoplasm, Jennings concluded:

“The direction of movement of particles on the outer surface is the same as that of the underlying particles of endosarc. The rate is also about the same as for the endosarc, though often, or perhaps usually, the outer particles move a little more slowly than those in the endosarc” (p. 142).

In view of the observations recorded in the preceding pages it is clear that Jennings’ statement that substance after moving forward on the upper surface, rolls over the anterior edge is quite erroneous. The attached particles, if heavy, may do so, but the surface film itself does not. It is, on the contrary, taken into the interior at the anterior edge.

The statement that the movement of the outer surface is congruent with that of the ectoplasm can likewise not be substantiated by observation, as has been demonstrated in the preceding pages. It is difficult to distinguish between the ectoplasm and the surface layer in such amebas as sphaeronucleosus and verrucosa, for there are no large crystals or other bodies which get caught in the ectoplasm as it is formed from endoplasm at the anterior end. But attentive observation will demonstrate very definitely that the ectoplasm here is stationary to the same degree as in proteus. The stationary properties of the ectoplasm are however not properly a matter for discussion; for five minutes’ observation of a proteus, discoides, annulata, particularly a laureata, under 300 diameters magnification, will convince anyone that the ectoplasm is stationary while the surface film, with attached particles, moves over it. No one can possibly come to any other conclusion. Jennings’ conclusion was due undoubtedly to an error of observation.

Jennings’ statement that the rate of movement of the outer surface is the same as that of the endoplasm (p. 142) when taken in connection with his other statement that the ectoplasm is a more or less permanent skin, presents a mechanical impossibility; for unless the outer surface moves twice as fast as the endoplasm, no rolling movement would be possible. Several of Jennings’ figures (especially Figures 38, 39, and 41) indicate in fact that he conceived of the outer surface as moving faster than the ameba advances, or that the upper surface moves over the ameba as the ameba moves over the substrate. Jennings’ theory requires that the surface layer move twice as fast as the ameba advances. Hyman (’17) also makes a similar mistake in referring to the rate of movement of the outer surface (p. 85).

Lest the discussion of this point be suspected of being merely verbalistic, it should be recalled that the surface layer of proteus often moves at about the same rate as the ameba; that the surface layer of discoides moves about twice as fast as the ameba; that the surface layer of verrucosa and sphaeronucleosus moves about three times as fast as the ameba; and that the ectoplasm does not move at all. It is of course incumbent on one to discuss what is stated; one is not at liberty to select one of several possible interpretations.

To illustrate this point graphically so as to avoid as far as possible future confusion Figure 29 is appended. In a is shown a particle traveling on an ameba at the same rate of speed as the ameba; at b is shown a particle that moves twice as fast as the ameba; at c the attached heavy particle does not move at all. For the sake of completeness d, Figure 29, is added here. It illustrates the backward moving ectoplasm in an ameba that is suspended in a jelly medium that prevents the ameba from sinking to the bottom. It must be admitted that in thus considering the rate of movement of the various tissues of the ameba from a single standpoint, a point outside of the ameba, little room is left for confusion.

There is comparatively little friction, if any at all, between the upper surface and the endosarc, according to Jennings’ view, since both these layers move at the same rate and as a single stream. On the other hand there must be very considerable friction between the endoplasm and the lower ectoplasm, which does not move at all. This difference in the amount of friction must show itself in the different speeds of the endoplasm near the upper ectoplasm and near the lower ectoplasm. Observation indicates however that the most rapid streaming of the endoplasm is in the middle of the ameba or pseudopod and that it gradually becomes slower as the ectoplasm is approached on all sides.

We said above that if the ectoplasm were a more or less permanent skin in which the ameba rolled as described by Jennings, the upper surface (=ectosarc, Jennings), according to a well known mechanical principle, would have to move ahead about twice as fast as the ameba advances. Now the upper surface of sphaeronucleosus and of verrucosa in locomotion was found to move from three to three and a half times as fast as the ameba (Chapter VII). In discussing movement in “verrucosa and its relatives” Jennings says “the essential features of the movement seem to be (1) the advance of a wave from the upper surface at the anterior edge; (2) the pull exercised by this wave on the remainder of the upper surface of the body, bringing it forward. Most of the other phenomena follow as consequences of these two” (p. 146). Thus the amount of stretch of the upper surface would exceed the amount of pull on it from 50% to 75%!

Jennings’ explanation of ameboid movement in which the important factor is a more or less permanent ectoplasm in which the ameba rolls along, would unquestionably produce rotary currents. Rhumbler (’98) recognized this and after full consideration rejected the idea that the ectoplasm is a permanent skin in which the ameba rolls along in locomotion, because rotary currents are not observed in a moving ameba. Anyone who doubts that rotary currents would be produced under these conditions can convince himself by putting a quantity of glycerine and some shavings in a large transparent rubber balloon or celloidin bag and letting it roll slowly down an incline in front of a strong light. If not too much glycerine is placed in the balloon, the shape of an ameba is closely enough approximated, and the rotary currents--down at the posterior and up at the anterior end--are well shown.

From all these considerations it is quite clear that Jennings’ explanation of ameboid movement as a rolling movement can not any longer be maintained. His “discussion of this matter (the rolling movement hypothesis) is an excellent example of the fact that acumen and excellent reasoning may lead one astray in scientific matters when the observational basis for the reasoning is not secure.” (These are Jennings’ own words in criticism of Rhumbler on the same subject!)

The surface tension theory, with its many modifications, has had a great many more adherents than any other theory that has been advanced to explain ameboid movement. It represents the attempt of biologists to explain a vital phenomenon on physical grounds. The fact that it has been held to go further in this direction than any other, and the fact of its greater simplicity, doubtless are responsible for its wider acceptance. The recent criticism to which this theory has been subjected, however, indicates clearly enough that this theory does not really give a very adequate idea of the processes involved in ameboid movement after all, and in so far as feeding processes are concerned, the theory does not seem to apply at all according to Schaeffer (’16, ’17). But it could hardly have been anything more than excellent guesswork if the surface tension theory as advanced by a number of writers had been found adequate, for the observational basis was very narrow, as the preceding pages have shown, and as the succeeding pages further show.

Not anything like a complete historical account of this theory with its numerous modifications will be attempted here. It would be a large undertaking, for nearly every biologist and biochemist has expressed himself on the subject. It does not appear that much is gained by merely recording the opinions, even of biologists, unless they are based on experimental or observational data, preferably their own. Scientific questions are not decided by ballot vote, and it is not apparent what value such a record of opinions would have except the doubtful one of showing whether the persons involved declared for or against the surface tension theory. Moreover such an account would not be interesting reading for those who want to know first of all what amebas can do. Only the more important modifications of the surface tension theory as applied to ameboid movements will therefore be discussed and these modifications will be considered important in proportion to the amount of observation or experiment on which they are based.

Attention has already been called in Chapter II to Berthold’s (’86) theory of ameboid movement, which was the first attempt to explain this phenomenon on physical principles. As will be remembered, Berthold thought that the nature of the ameba’s immediate environment determined when and in what direction it should move, the source of the energy of movement being supposed to be a decrease in the tension of the surface film of the ameba, brought about by some factor in the ameba’s immediate environment.

One of the most elaborate attempts that has been made toward explaining ameboid movement on the basis of surface tension phenomena was that of Bütschli (’92). From his extensive knowledge of the lower organisms, especially the protozoa, he concluded that protoplasm is an emulsion of two fluids: a more concentrated “plasma,” insoluble in water; and a thinner fluid, “enchylema.” Ameboid movement was brought about by migration of enchylema droplets to the surface of the ameba at the anterior end, where they burst and spread over the surface, lowering its tension. The effect of this change in tension was held to be a flowing backward of the surface of the ameba and a flowing forward of the endoplasm. This is what happens in a drop of fluid, such as oil, on water to one side of which is brought a soapy solution. Bütschli described many experiments with fluids on which the surface tension was changed by appropriate means to simulate the process of movement. After Bütschli had developed his surface tension theory of movement, he discovered, as has already been noted, that in a pelomyxa the surface layer moves forward instead of backward as required by the surface tension theory. In spite of this however he still maintained that his theory of movement could be modified to apply to amebas generally, although so far as I have been able to find, he did not then or subsequently state how. From this we may infer that Bütschli himself probably concluded that the surface tension theory of movement as he developed it, is not of general application or is nothing more than a step in the development of such a theory.

Rhumbler has written a number of papers on the mechanics of ameboid movement, most of which are concerned with elaborations and modifications of a surface tension theory very similar to Bütschli’s. Rhumbler published a general outline of his theory in 1898. The transformation of endoplasm into ectoplasm at the anterior end, and the reverse process at the posterior end, was stated to be an important part of his theory of movement, but just how this was necessary to surface tension effects was not explained in physical terms. Feeding was assumed to be caused by the direct action of the food body on the surface layer (ectoplasm) of the ameba. The presence of the food body, he held, produced a lowering of the surface tension of the ameba thus causing the ameba to flow around it (’98, p. 207). Subsequently, however, he (’14) came to the conclusion that many amebas cannot have fluid surfaces as usually understood, since they do not spread as a film over water when they come into contact with the surface. From this and other observations Rhumbler concluded (’14, pp. 501-514) that the surfaces of amebas are not to be compared with surface tension films on drops of inert simple fluids; but with the surface films of emulsions which take on the properties of a solid. Since the question of ameboid movement is not especially discussed in this later paper, it may be assumed that in this respect his (’98, ’10) earlier views have not been materially modified. Rhumbler has suggested a great many physical models for the explanation of various ameboid activities such as feeding, defecation, movement and so forth.

In general agreement with Bütschli and Rhumbler were Verworn (’92), Blochmann (’94), Bernstein (’00), Jensen (’01, ’02), and recently Hirschfeld (’09) and McClendon (’12). All these authors held that ameboid movement is a surface tension phenomenon. The application of the surface tension theory in explaining ameboid movement demands a fluid surface and a fluid interior and it is perhaps unnecessary to add that Bütschli, Rhumbler and the others mentioned held that the protoplasm is fluid. The question as to whether protoplasm is a fluid or possessed of an internal structure was however hotly debated and we find Fleming (’96), Heidenhain (’98), Klemensievicz (’98), Dellinger (’06) and others opposing the group of authors just mentioned, by contending that the streaming protoplasm must have some kind of structure. This question no longer concerns us however, owing to our rapidly increasing knowledge of colloidal solutions, for it is undoubtedly correct to hold that protoplasm is colloidal.

We have already insisted (p. 46) that the problem of ameboid movement is made more difficult by narrowing it down to the movements of ameba, and that to see the problem in its fullest aspect requires consideration of streaming protoplasm wherever found. Now it happens that there is in certain respects greater diversity of streaming to be found in plant cells than in animal cells, and it is not surprising therefore that explanations of streaming and ameboid movement have taken a different direction among botanists than among zoologists. It is for this reason doubtless that Ewart (’03), while espousing the surface tension theory as explaining streaming, does not look to the superficial surface of a plant cell as the source of the necessary energy, but to the interior of the protoplasm. This idea is, of course not entirely original with Ewart, for Bütschli, as we saw, believed that protoplasm has an emulsoid structure; but according to Bütschli’s hypothesis, the surface forces were not brought into play in movement until the droplets of enchylema spread over the surface and so reduced the tension. Ewart, however points out that there is very much more surface energy present in the interior of streaming protoplasm than is required for all the movements known to protoplasm, including muscular contraction. According to Ewart’s hypothesis the emulsion globules (disperse phase) have their surface tension lowered at corresponding points by electrical currents traversing the endoplasm, the electrical currents themselves originating in chemical actions.

While all available evidence from the study of colloidal solutions and from observation from protoplasm confirms Ewart’s statement that more than sufficient energy is available in the interior of colloids for all purposes of movement, there is little or no evidence that the proper electrical currents are present to release or transform the surface energy into that of movement. This step in his explanation is therefore highly hypothetical and at present unconvincing. Moreover, this step in the theory would not be applicable to streaming as observed in amebas, without very considerable modification.

Recently Hyman (’17) has developed the surface tension theory of movement in the direction indicated by Ewart. The motive power is supposed to have its source in the contractility of the ectoplasm. The endoplasm is held to be a passive stream, not an active stream as Ewart supposed to be the case in plant cells. The power of contractility is held to be due to the process of gelation of endoplasm into ectoplasm, which is due to a change of phase, the fluid part of the endoplasm becoming dispersed and thereby developing surface energy in proportion as the amount of surface of the fluid is increased. This increase of surface produces the phenomenon of contractility.

Miss Hyman is wrong however when she says (p. 90) that the withdrawal and contraction of pseudopods are processes of gelation. This is clearly a physical impossibility, for the ectoplasm of the withdrawing pseudopod must become liquified into endoplasm, before it can be withdrawn. All writers excepting Jennings and Hyman are agreed on the continual transformation of ectoplasm into endoplasm at the posterior end while the reverse process goes on at the anterior end; and Hyman herself states (p. 89) that new ectoplasm is formed as the growing pseudopods extend into the water. So there must be liquefaction of the ectoplasm in withdrawing pseudopods, or very soon the whole ameba would be transformed into ectoplasm. As was shown in the preceding pages, liquefaction of the ectoplasm at the posterior end goes on at the same rate as gelation of the endoplasm at the anterior end. But at another place Hyman says:

“In fact according to Jennings, Dellinger, Gruber, and Schaeffer the surface of the ectoplasm actually flows forward at about the same rate as the forward advance, and this indicates that the advancing ectoplasm at the tip of the pseudopodium is derived from the surface ectoplasm and not from a transformation of endoplasm into ectoplasm at the end of the pseudopodium as Rhumbler supposed” (p. 89).

This quotation is not strictly accurate. Jennings says: “The pseudopodium grows chiefly from the base, so that any part of the surface retains nearly its original distance from the tip” (p. 156). Dellinger in a general way confirmed Jennings’ conclusions. Gruber concluded that the outer layer was gelatinous, not protoplasmic. Schaeffer held the third layer to be extremely thin, “too thin to be seen easily,” so it is impossible that the ectoplasm at the tip of a pseudopod, the thickness of which is readily seen, can be derived from the surface film.

The main conclusion however in Miss Hyman’s paper is that there exists a metabolic gradient in the pseudopods of advancing amebas, the highest rate of metabolism being at the tip and the lowest at the base, for any one pseudopod. This conclusion is bound to be of the first importance in the explanation of ameboid movement. It will give our first real insight into the chemistry of ameboid movement. The fact that her method of demonstrating gradients has yielded uniform results in the hands of Child (’15), who originated it, as well as in her own when applied to a great many different organisms, entitles her conclusions to careful examination.

Of the observations there can be no doubt, for in many details earlier observations are confirmed. Her figures show that the tips of the pseudopods disintegrate first in the potassium cyanide solution and later the regions further back (Figure 30). The question is, what causes the gradient of disintegration, which Miss Hyman takes to represent also a metabolic gradient? Where is the gradient located: in the ectoplasm or in the endoplasm; or is the gelation process synonymous with the metabolism that gives rise to the observed gradient? Miss Hyman does not say; but it cannot be in the endoplasm, for it is in motion along the whole pseudopod at about the same rate and it undergoes a demonstrable and visible change only at the anterior end of the pseudopod. While metabolic changes might be higher at the free end of the pseudopod, therefore, there would not be a gradient from there on back. No recorded observations on the endoplasm along the length of a pseudopod can be arranged so as to form a gradient which would suggest a similar gradient in metabolic rate; and if the endoplasm is a passively moved fluid as Hyman’s theory seems to imply, a metabolic gradient would seem to be precluded.

In the ectoplasm however there exists a time gradient; that at the base of a pseudopod is older than that near the tip, and observation generally tends to confirm the view that the older it is the firmer it becomes. This gradient in the amount or extent of gelation corresponds with the disintegration gradient of cyanide along a forming pseudopod. That is, the rate of disintegration is proportional to the age of the ectoplasm. There is however no good evidence that the age of ectoplasm corresponds to the rate of metabolism, so that the younger the ectoplasm is the higher will be the metabolic rate in it. The following statement seems to bear this out: “As soon as the pseudopodium extends into the water its surfaces gelatinizes because of contact with the water” (Hyman, ’17, p. 89). Gelation is, according to Hyman, a passive process and therefore not distinctively metabolic. She continues: “It is necessary therefore for the continuous production of a pseudopodium, that the metabolic change which is the cause of the liquefaction should continue to occur at the pseudopodial tip. There is thus produced the metabolic gradient along the pseudopodium which I have described....”

But if the metabolic gradient is bound up with the process of liquefaction, it is difficult to see how there can be a gradient along the pseudopod, for liquefaction takes place only at the tip, according to her own statement. As a matter of fact, however, gelation is constantly occurring at the tip of the pseudopod and to a less degree back along the sides of the pseudopod. Liquefaction occurs only at the posterior end of the ameba in orderly movement.

We must conclude therefore that while Hyman’s data are of the first importance in contributing to the structure and behavior of the ameba, her contention that a metabolic gradient is demonstrated in the ameba is not convincing.

From this short account of the main theories that have been advanced to explain ameboid movement it appears that of the modern theories the only one that has been capable of adjusting itself to new investigations and observations is the surface tension theory. The earlier theories under this head were mistaken however in looking to the superficial film of the ameba as the source of energy. But with the increase in knowledge of the chemistry of colloids, the source of the surface energy came to be located in the interfaces between the phases of the colloidal system. As has already been remarked, there is more than sufficient free energy here to account for all the movements observed in protoplasm; there remains the problem of explaining how the surface energy is transformed into that of movement. As Graham (’61) remarked: “The colloidal is in fact a dynamical state of matter. The colloid possesses energia. It may be looked upon as the probable primary source of the force (energy) appearing in the phenomena of vitality.”

Now, viewing streaming wherever it occurs in the protoplasm of animals or plant cells the surface tension theory, as far as observations permit, applies to the various conditions of streaming as follows.

In the first place we shall begin with the assumption that is generally held, that protoplasm is a reversible colloidal solution consisting mainly of proteins, with some carbohydrates, lipoids, etc., on the one hand and water on the other. Its reversibility consists of course in being able to change from a sol to a gel state and the reverse, the water being in the disperse phase in the gel state. The consistency of the protoplasm therefore depends upon two factors: upon the amount of water present, and upon the degree of its dispersion; the smaller the droplets the more solid will be the gel because of the increase in surface of the mass. Colloids exhibit the property of contractility in proportion as the droplets of water are decreased in size; or, which amounts to the same thing, in proportion as the amount of the surface of the water is increased. It appears as if the source of energy of contractility was the free energy in the surface films of the internal phase of the gel.

Taking the amebas as a group and applying these principles of colloidal solutions, we find that we can arrange the amebas in a series of four or more grades representing differences of fluidity of the protoplasm. Among the most fluid are Amoeba limicola and Pelomyxa schiedti; in the next group, with less fluid protoplasm is Amoeba dubia; in the third group is A. proteus and A. discoides; in the fourth group, with the least fluid protoplasm, come A. radiosa and A. verrucosa. These groups represent a progressive increase in the amount of ectoplasm in proportion to the endoplasm. There being less water present in the higher groups than in the lower, which follows from a stiffer endoplasm, it is possible for them to form endoplasm, that is, to change phase, more readily. And as a corollary to this we may add that more pseudopods are formed, since ectoplasm can be formed more readily. (The verrucosa types possess very stiff ectoplasm, and they increase their surface by flattening out and by forming longitudinal ridges. They cannot for some unknown reason form pseudopods). Again with the increase in the consistency of the protoplasm, the pseudopods become more slender (and stiffer) and more contractile, the most slender pseudopods (radiosa, flagellipodia) being very much more contractile than the larger ones of proteus or discoides, for example. An additional factor operates here, however, for some of the slender pseudopods as of radiosa and bilzi are static and for a great part of their existence practically incapable of contraction. The high development of contractility follows, of course, from the high degree of dispersion of the internal phase in ectoplasm, of which these pseudopods almost wholly consist. Thus, many, if not most, of the more generalized peculiarities of form of amebas may be traced to the amount of water in the protoplasm.

The number of pseudopods in an ameba is an important factor in its method of locomotion, as may readily be perceived. Since amebas generally move with great variation in speed as one compares the different species, whether they form very little ectoplasm or very much, and are able to maintain themselves on their paths, it follows that ectoplasm formation by itself does not play an important part in originating movement. But it requires only a few minutes’ observation to see that ectoplasm is necessary to guide the ameba, so to speak, and to make the endoplasmic stream effective for the purpose of orderly movement. It requires very little imagination to see what would happen if no ectoplasm were present in a limicola or any very fluid ameba. Streaming would undoubtedly occur as before, but the currents would be rotational and irregular and no progression could take place. The ectoplasm furnishes just that stiff tube against which the backward action of the endoplasm can impinge so to speak in order to enable it to flow forward. The ectoplasm is essential for orderly movement forward, but it is not essential for streaming.

But this does not imply that the contractile power of the ectoplasm may not be used to aid in propelling the endoplasm in streaming. It has been demonstrated by Miss Hyman (’17) that the ectoplasm is actually contractile when the ameba is strongly stimulated all over its exterior by a solution of potassium cyanide. While this proves only the contractile powers of the ectoplasm under exceptional conditions, and when at rest, it is not impossible that under ordinary conditions of locomotion it may aid in streaming. There is however one observation which may, upon further investigation, negative this possibility. Frequently in a pseudopod about to be retracted some of the endoplasm flows toward the tip while the rest flows toward the base (Figure 1, p. 11).

One more point needs mention in this connection, and that is the small waves of clear protoplasm which are thrown out by many amebas at their anterior ends during locomotion. They are especially prominent in A. bigemma (Figure 7) and in radiosa (Figure 8), but they are formed in perhaps all species. Observation does not indicate that they move in exactly the same way as the main body of the endoplasm, even if the larger granules could be left out of account. They behave more like the clear pseudopods of Difflugia and Arcella and the foraminifera.

Although these waves are frequently not to be seen during locomotion in Amoeba proteus and other large amebas, particularly in Pelomyxa palustris and P. belevskii, it is possible that the wave forming process has become indistinguishably merged with endoplasmic streaming. It is not impossible that the projection of these waves is the purest expression of ameboid movement. But on account of their small size and transparency, it is very much more difficult to investigate them than streaming of the granular endoplasm, as it is observed in amebas, ciliates and plant cells. It seems to be true however that streaming can occur in the entire absence of these waves, so their importance in ameboid movement is probably secondary.