Figure 29.a, a particle attached to an ameba and moving at the same rate as the ameba. This condition is often observed inproteuswhere the surface film, owing to its destruction during the formation of the longitudinal ridges, retards the forward movement of this layer.b, a particle attached to the surface film of an ameba moving twice as fast as the ameba. This condition is seen indiscoides,verrucosa,sphaeronucleosus, etc.c, a particle on an ameba that does not move at all although the ameba does. This is seen when a heavy particle is laid on an ameba, too heavy for the surface film to move.d, movement of ectoplasm in an ameba suspended in a jelly medium. The vertical lines are to be considered as stationary.
Figure 29.a, a particle attached to an ameba and moving at the same rate as the ameba. This condition is often observed inproteuswhere the surface film, owing to its destruction during the formation of the longitudinal ridges, retards the forward movement of this layer.b, a particle attached to the surface film of an ameba moving twice as fast as the ameba. This condition is seen indiscoides,verrucosa,sphaeronucleosus, etc.c, a particle on an ameba that does not move at all although the ameba does. This is seen when a heavy particle is laid on an ameba, too heavy for the surface film to move.d, movement of ectoplasm in an ameba suspended in a jelly medium. The vertical lines are to be considered as stationary.
Figure 29.a, a particle attached to an ameba and moving at the same rate as the ameba. This condition is often observed inproteuswhere the surface film, owing to its destruction during the formation of the longitudinal ridges, retards the forward movement of this layer.b, a particle attached to the surface film of an ameba moving twice as fast as the ameba. This condition is seen indiscoides,verrucosa,sphaeronucleosus, etc.c, a particle on an ameba that does not move at all although the ameba does. This is seen when a heavy particle is laid on an ameba, too heavy for the surface film to move.d, movement of ectoplasm in an ameba suspended in a jelly medium. The vertical lines are to be considered as stationary.
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 onall 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 surfaceofsphaeronucleosusand ofverrucosain locomotion was found to move from three to three and a half times as fast as the ameba (Chapter VII). In discussing movement in “verrucosaand 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 ofstretchof the upper surface would exceed the amount ofpullon 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 thisdirection 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 theoryas advancedby 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 animalcells, 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 amountof 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 ameboidmovement. 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.
Figure 30. Disintegration of an ameba in ¼ molecular KNC. After Hyman.a, ameba flowing in the direction of the arrow.b, the ameba has abandoned pseudopod 1 and flows into pseudopod 2, which has become reactivated. The ameba was exposed to KNC at this stage and, as is usual in such experiments, the posterior end atxbecomes active.c, the youngest pseudopod, atx, disintegrated first.d, the next youngest pseudopod, 2, disintegrated next. Pseudopod 1, the oldest, disintegrated last.
Figure 30. Disintegration of an ameba in ¼ molecular KNC. After Hyman.a, ameba flowing in the direction of the arrow.b, the ameba has abandoned pseudopod 1 and flows into pseudopod 2, which has become reactivated. The ameba was exposed to KNC at this stage and, as is usual in such experiments, the posterior end atxbecomes active.c, the youngest pseudopod, atx, disintegrated first.d, the next youngest pseudopod, 2, disintegrated next. Pseudopod 1, the oldest, disintegrated last.
Figure 30. Disintegration of an ameba in ¼ molecular KNC. After Hyman.a, ameba flowing in the direction of the arrow.b, the ameba has abandoned pseudopod 1 and flows into pseudopod 2, which has become reactivated. The ameba was exposed to KNC at this stage and, as is usual in such experiments, the posterior end atxbecomes active.c, the youngest pseudopod, atx, disintegrated first.d, the next youngest pseudopod, 2, disintegrated next. Pseudopod 1, the oldest, disintegrated last.
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 forma 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 agradientalong the pseudopod, for liquefaction takes place only at the tip, according to her own statement. As a matter of fact, however,gelationis 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 adjustingitself 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 possessesenergia. 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 areAmoeba limicolaandPelomyxa schiedti; in the next group, with less fluid protoplasm isAmoeba dubia; in the third group isA. proteusandA. discoides; in the fourth group, with the least fluid protoplasm, comeA. radiosaandA. 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. (Theverrucosatypes 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 ofproteusordiscoides, for example. An additional factor operates here, however, for some of the slender pseudopods as ofradiosaandbilziare 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 alimicolaor any very fluid ameba. Streaming would undoubtedly occur as before, but the currentswould 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 inA. bigemma(Figure 7) and inradiosa(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 ofDifflugiaandArcellaand the foraminifera.
Although these waves are frequently not to be seen during locomotion inAmoeba proteusand other large amebas, particularly inPelomyxa palustrisandP. 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.
The nearest relatives of the amebas are the shelled rhizopods, the Difflugias and the Arcellas and their congeners. The movement of these organisms is quite different from that of the amebas in that the whole body of the endoplasm does not stream into the pseudopods, but only a small portion of it. There is consequently no regular transformation of ectoplasm into endoplasm at the posterior end, that is, the protoplasmic mass within the shell. The method of movement inDifflugiawas described by Dellinger (’06). A pseudopod is thrown out to a considerable distance. It fastens itself to the substrate at the tip. It then contracts, pulling theDifflugiaforwards. While this pseudopod is contracting, another one is extended in the same direction. When it has arrived at the maximum length, it fastens itself at the tip and then contracts, pulling theDifflugiaalong. Continued locomotion consists of a repetition of this process. The pseudopods are slender and consist nearly always of clear protoplasm. Only occasionally does one see conspicuous endoplasmic granules flow into a pseudopod, and then only at the base.
The transparency of the pseudopods inDifflugiaand the absence of granules in the protoplasm composing them, prevents one from seeing clearly how the pseudopods are formed, that is, whether or not there is a regular transformation of endoplasm into ectoplasm at the anterior end. The fact that one occasionally sees the endoplasm stream into the base of a pseudopod in the same way as was described for ameban pseudopods, indicates that the method of formation of pseudopods inDifflugiais in general similar to that in ameba. But the process is not exactly the same, for the surface layer on the pseudopods ofDifflugiadoes not move as fast as the tips of the pseudopods advance, while in amebas the surface layer moves faster than the pseudopods. What this difference indicates has not yet been ascertained.
The protoplasm of the pseudopods ofDifflugiais thick andthe power of contractility highly developed, for the pseudopods readily move about in the water like a tentacle. The demarcation line between ectoplasm and endoplasm is very difficult to see, consequently no definite idea can be given as to the thickness of the ectoplasm. When a pseudopod is being extended the whole contents seem to move at about the same rate as the pseudopod advances, differing thus from amebas, in the pseudopods of which the central core of the endoplasmic stream flows considerably faster than the tip of the pseudopod advances through the water. But when a large pseudopod is cut off from aDifflugiait is able to move after the manner of an ameba without a nucleus (Verworn, ’94).
In heliozoans protoplasmic streaming is quite different from that in ameba orDifflugia. The pseudopods are usually straight, radiating from the central body. They possess usually a central axial rod of condensed or strongly gelatinized protoplasm around which is a layer of thick protoplasm with the properties of ectoplasm. Heliozoans for the most part move slowly; in fact many of them are pelagic and in these the power of locomotion on a solid substratum is very slow. There is however one species,Acanthocystis ludibunda, which, according to Penard (’04), can move twenty times its diameter in one minute by rolling. This illustrates a highly developed power of contractility in the pseudopods of this organism, for since only about one-fifth of the circumference can be in contact with the solid substratum, the pseudopods must attach themselves, contract so as to pull theAcanthocystisalong, and relax their hold, all in the space of two seconds.
Among pseudopod forming organisms, the highest development of contractility is found in the foraminifera. As is well known, these organisms form finely anastomosing pseudopods which frequently cover the substratum with a network of protoplasmic strands. The terminal sections of these strands are frequently so thin and transparent that they cannot be seen easily with the microscope. As a rule the granular endoplasm is observable only in the main body of the organism and in the larger trunks of the pseudopods. Much the larger part of the pseudopods, as measured lineally, is devoid of granular endoplasm. Thegreat power of contractility and the speed with which contraction may occur inBiomyxa, a fresh water foraminifer, have already been mentioned (Figure 12, p. 47). Similar observations have been recorded by other observers, recently by Schultz (’15), who compares the contractility of foraminiferan pseudopods to that of rubber bands. In fact as one watches the movements of aBiomyxa, for example, under moderately high magnification, one gains the impression that there seems to be no restriction imposed upon the extent of contractility in the pseudopods. They seem to possess perfect elasticity. As to the transformation of endoplasm into ectoplasm, little can be said, owing to the transparency of the protoplasm. But the whole of the pseudopod, when forming, seems to stream forward. As inDifflugia, the interior streams flow at about the same rate as the pseudopod as a whole advances. The highly developed power of contractility however demands rapid changes in phase of the colloidal system, and also a thick consistency. The behavior of pieces of the pseudopodial network, when cut from aBiomyxa, shows clearly that the protoplasm is actually thick, as compared with that of anAmoeba proteus. When aBiomyxais contracted into a spherical mass, the interior exhibits continual rapidly streaming movements. Some of these are rotational but most of them are radial. All of the streams frequently change their direction and extent. No corresponding changes are visible in the outer peripheral layer.
Among plants, some of the algae possess ameboid protoplasts at one stage or another of their life cycle, but the details of streaming have not been made out. It has been reported however that the zoospores of some parasitic fungi move to all appearances exactly like small amebas. We likewise lack details of the streaming of the myxomycete plasmodia. From a more or less cursory examination of a small aquatic plasmodium of undetermined species, it appeared that the formation of pseudopods and the process of streaming were quite different from similar processes in the foraminifera. The pseudopods do not act independently as in foraminifera. At almost the same moment the protoplasm begins to flow from the pseudopods in a large section of the plasmodium and into another section; then soon thereafter the protoplasm flows back again. This oscillatory streaming is continuedpresumably as long as the myxomycete is in the plasmodial stage. With every change in the direction of movement of streaming, there is produced, however, a change in the shapes of the pseudopods, so that with a number of oscillations in streaming an appreciable degree of locomotion is effected. The direction of locomotion can be markedly affected by changes in light intensity and moisture distribution, as shown by the observations of Baranetzsky (’76), Stahl (’84) and others, but just how these changes in the direction of locomotion were produced is not recorded. There is a definite ectoplasm and a definite endoplasm in the myxomycete plasmodia, but the details of their transformations, the one into the other, have not been determined; but since the surface layer is stationary, it is probable that there is no such regular transformation of endoplasm into ectoplasm at the anterior ends of pseudopods as there is in ameba. But this phase of the subject needs further investigation before any conclusions can be drawn. The power of contractility is present, but apparently only to a slight degree. Too little is known of the streaming process in these organisms to compare it in detail with the same phenomenon in rhizopods.
The streaming of protoplasm in plants has received a good deal of attention, though only comparatively little experimental work has been done. Streaming is observed in a great many plant cells, and in some cells such as the large internodal cells ofCharaandNitella, the process may be easily observed. The essential features of a plant cell in which streaming occurs are, first, the external cell wall of cellulose, which of course prevents any change of shape in the cell such as is observed in naked protoplasts as, for example, ameba. Inside of the cell wall is a layer of ectoplasm which has essentially the same properties as the ectoplasm of amebas. In some cells such as those ofChara, the ectoplasmic layer is thick and contains nearly all the chloroplastids, while in the leaf cells ofElodeathe ectoplasm is extremely thin and is practically free from chloroplastids. In the interior of the cell are found the streaming endoplasm and one or more large vacuoles filled with cell sap.
The streaming is of two types which are often distinguished from each other by the namesrotationalandcirculatory. But thedistinction seems to be of little significance, for the same cell may at different times show both types of streaming. When there is a single vacuole only in the cell, it occupies the center of the cell, and the endoplasm then rotates between it and the ectoplasm. Whenever there are strands of endoplasm flowing across the vacuole, the peripheral streaming is no longer rotational but it is then called circulatory. By external stimulation of the cell, Ewart (’03) was able to change circulatory streaming into rotational; that is, the numerous small streams traversing the cell sap in many directions were caused to retract into a single stream around the periphery of the cell. This change brought about a heightened velocity in streaming, showing that the small strands traversing the cell sap meet with some resistance. There is no essential difference between streaming in plant cells, whether rotational or circulatory, from the rotational streaming so commonly found in protozoa.
Figure 31. Diagram of a section of aCharacell showing rows of emulsion globules in the endoplasm, after Ewart.a, cell wall.b, ectoplasm.c, endoplasm,d, cell sap. The arrows at the top of the figure indicate by their lengths, the amount of movement of the endoplasm and cell sap in streaming.
Figure 31. Diagram of a section of aCharacell showing rows of emulsion globules in the endoplasm, after Ewart.a, cell wall.b, ectoplasm.c, endoplasm,d, cell sap. The arrows at the top of the figure indicate by their lengths, the amount of movement of the endoplasm and cell sap in streaming.
Figure 31. Diagram of a section of aCharacell showing rows of emulsion globules in the endoplasm, after Ewart.a, cell wall.b, ectoplasm.c, endoplasm,d, cell sap. The arrows at the top of the figure indicate by their lengths, the amount of movement of the endoplasm and cell sap in streaming.
Ewart has also observed that in the streaming of the endoplasm, there is a variation of velocity of streaming in different parts of the stream (Figure 31). The middle of the stream moves fastest while the layer near the ectoplasm moves very slowly and the layer in contact with the ectoplasm moves hardly at all. But the endoplasm in contact with the central vacuole moves only a little moreslowly than the middle of the stream, and the effect of this is that the outer edge of the vacuole is dragged along with the moving endoplasm. This is an important observation and from it Ewart concludes that the energy which produces the streaming movement must be liberated, not at the boundary between the ectoplasm and the endoplasm, nor at that between the endoplasm and the vacuole, but within the endoplasmic stream itself. In this conclusion Ewart is undoubtedly correct, for as a physical phenomenon, no other conclusion is at present possible.
Other experiments made upon the velocity of streaming in plant cells indicate that the streaming process obeys the laws of physics. The velocity varies with the proportion of water present in the endoplasm,—the more water, the faster the streaming (Ewart, ’03). The effect of temperature on streaming, noted first by Corti (’74), and studied by Velten, (’76), Schaeffer (’98), Ewart (’03) and other writers, is also such as would be expected if the endoplasm were a simple physical fluid.
The rotational streaming in plant cells, such as those ofChara, is very similar to the rotational streaming in paramecium and numerous other ciliates. In these organisms it is often called cyclosis. A paramecium differs, however, from a plant cell exhibiting rotational streaming in that no central vacuole is present. This space in paramecium is occupied by the gullet, the nucleus and some endoplasm which is not in the main stream. The effect of this difference seems to be one affecting velocity only, slowing it down, for in theCharacell the endoplasm meets with much less friction when moving in contact with the vacuolar wall than when moving in contact with the ectoplasm. Its velocity is still further reduced by the large food vacuoles which are almost always carried by the endoplasm, for these vacuoles behave like solid bodies in the endoplasmic stream. During streaming these vacuoles are often seen coming close to the limiting ectoplasm, when they act as obstructions to the flow of the endoplasm. The velocity of the endoplasmic stream in paramecium is relatively slow, ten to twenty minutes being required for a complete revolution.
InFrontonia leucas, another large ciliate, rotational streaming is under the control of the organism, and special use is made of itin feeding.Frontoniafeeds mostly, if not entirely, on large particles. It has no oral groove like paramecium has, and when swimming no ciliary vortex is produced such as is seen in paramecium.Frontoniafeeds mostly by “browsing,” that is by eating particles lying on or against some solid support, though it is able also to feed upon particles suspended in the water.
OscillatoriaandLyngbiaand other filamentous algae are the chief food ofFrontonia. Filaments of these algae are ingested by pulling them into the mouth and then rolling them up into a coil in the body. Pieces ofOscillatoriasix to eight times as long as theFrontoniaare readily eaten in this way.
As a rule the end of a filament is seized by the mouth and gradually passed back into the body (Figure 32,a). As soon as the tip of the filament is well in the mouth and in contact with the endoplasm, streaming begins in the endoplasm in the region of the mouth and takes a direction directly back against the aboral wall, almost, if not quite perpendicular to the longitudinal axis. This stream of endoplasm carries the filament back to the aboral wall, sometimes pushing out the wall a considerable distance. Presently, however, the filament is carried posteriorly along the aboral wall by the streaming protoplasm, which has by this time become rotational, and after reaching the posterior end the filament is brought up along the oral wall. The rotational streaming continues until the entire filament is wound up, which in exceptional cases may make four or five coils inside the animal.
The mouth has considerable grasping power. This is shown inFigure 32where a filament ofOscillatoriawas bent upon itself by the mouth and then rolled up in the body by the endoplasm in the same manner as a single filament. The mere viscosity of the endoplasm would be insufficient to bring about the bending of the filament. For the sake of comparison it should be added that a similar grasping power is also present in paramecium. The moment the food vacuole at the mouth is large enough, the endoplasm pulls it away and moves it rapidly toward the posterior end of the paramecium, much more rapidly than it would be carried by the rotationally streaming endoplasm. But from the posterior end forward the food vacuole is carried at the same rate as are the other particles in the endoplasm. In bothFrontoniaand paramecium rapid endoplasmic streaming precedes for a short distance the forward end of the ingested filament or the food vacuole (Figure 32,a).
Figure 32. Showing ingestion of alga filaments inFrontonia leucas.a, the beginning of the ingestion of an alga filament. Note the streaming of the endoplasmprecedingthe end of the filament.b, almost two complete coils of the filament have been rolled up inside theFrontoniaby the rotary streaming endoplasm. The endoplasm in the center of the animal is stationary.c, a filament, if thin, may be grasped anywhere along its length, bent together and swallowed in the usual manner. Diameter ofa, 250 microns.
Figure 32. Showing ingestion of alga filaments inFrontonia leucas.a, the beginning of the ingestion of an alga filament. Note the streaming of the endoplasmprecedingthe end of the filament.b, almost two complete coils of the filament have been rolled up inside theFrontoniaby the rotary streaming endoplasm. The endoplasm in the center of the animal is stationary.c, a filament, if thin, may be grasped anywhere along its length, bent together and swallowed in the usual manner. Diameter ofa, 250 microns.
Figure 32. Showing ingestion of alga filaments inFrontonia leucas.a, the beginning of the ingestion of an alga filament. Note the streaming of the endoplasmprecedingthe end of the filament.b, almost two complete coils of the filament have been rolled up inside theFrontoniaby the rotary streaming endoplasm. The endoplasm in the center of the animal is stationary.c, a filament, if thin, may be grasped anywhere along its length, bent together and swallowed in the usual manner. Diameter ofa, 250 microns.
If a filament of alga is too long for theFrontonia, or one end of it is fast, streaming is reversed after several coils have been rolled up and the filament is ejected. So far as could be observed, the streaming process is reversed in all details, though the rate of ejection seemed to be somewhat slower than the rate of ingestion. Occasionally, however, ejection is accomplished much more quickly. If there are several coils of a filament whose other end is fast, rolled up inside of aFrontonia, the mouth sometimes stretches antero-posteriorly until the coil as a whole without unwinding is thrown out of the body. The viscosity of the endoplasm might lead one to expect that some of the endoplasm would be brought out with the alga, but such is not the case.
The essential differences between rotational streaming inFrontoniaand in paramecium are: (1) It is under the control of the organism inFrontoniawhile in paramecium it is a continuous reversible process. (2) It is much more rapid inFrontoniathan in paramecium. On the other hand, the physics of streaming inboth organisms is essentially the same, so far as could be detected. In both organisms the energy of streaming is liberated within the endoplasm. This is especially well shown in the first stages of feeding.
Besides these organisms in which streaming occurs, either in a part of the organism or the whole, streaming is also found to occur in a great variety of plants other than those already mentioned; in the leukocytes of perhaps all coelomates; in some animal egg cells, such as the sponges, hydra and molluscs; in pigment cells, especially in batrachians and lacertilians; in phagocytes and wandering cells of a great many animals; in the nuclei of some animal cells; and in the intestinal epithelial cells of perhaps all metazoans. In almost none of these cases however do we know more than the bare fact that streaming occurs. No details are known. Consequently in so far as the purposes of this book are concerned it will not be apropos to discuss these cases further except to record the thesis that there is no evidence tending to show that these cases are not at bottom all characterized by the operation of the same fundamental process.
In all these cases of animal and plant cells and tissues in which ameboid movement occurs the process of streaming is easily observed in all of them, but the phenomenon of contractility is not noticeable in some cells except under special conditions, while in other cells it is operating continually. This indicates that there are other factors at work in addition to mere phase changes in the colloidal system to produce now contractility, now streaming. A high power of contractility and of streaming are not present in the same mass of protoplasm at the same time, though these powers may both be present at different times (Biomyxa).
Contractility can be explained in a general (though not yet in a detailed) way as due to a change in phase, more or less complete, in the colloidal system which is held to be the chief characteristic of the physical aspect of protoplasm. The change of phase is of course, associated with a change in the amount of surface energy, which is the ultimate source of the energy of contractility.
Streaming, however, does not depend upon amarkedchange ofphase resulting in gelation, for observation has failed to detect this process going on to any extent whatever in streaming protoplasm. Further, an increase in the amount of water in the protoplasm is associated with more rapid streaming. If streaming therefore depends upon a phase change in a colloidal system, it must be in the direction of liquefaction, that is, changing the internal more fluid phase to the external phase. A phase change in one direction would thus lead to contractility, while a change in the other direction would lead to streaming.
Theories accounting for the intimate nature of the process of streaming without special reference to ameboid movement, have been offered by many botanists. In most plant cells in which streaming movements occur the ectoplasmic covering does not change shape. Streaming of the endoplasm therefore is a much less complicated process in such a case than in an ameba where locomotion is also present. It is to be expected therefore that a theory of streaming based upon observation of a plant cell such as is found inCharawould be different from one based upon observation of a moving ameba. Such is found to be the case, as the following discussion of some of the principal theories accounting for streaming in plant cells strikingly shows.
(1)The contractility theories. Corti (’74), who was the first to record observations on the process of streaming in plants thought that the movement of the endoplasm was caused by waves of contraction passing around the cell in a way analogous to that in which fluid may be passed through a rubber tube by closing the finger over it and passing it along the tube. Heidenhain (’63), Kühne (’64), Brücke (’64), Hanstein (’80), in one form or another also have expressed their adherence to the contractility theory. More recently Dellinger (’06, p. 356) postulated contractile fibrillae in rhizopods similar to those postulated by Brücke to explain protoplasmic streaming. The contractility theories are no longer considered tenable, for no waves of contractility can be demonstrated, as the theories of Corti, Heidenhain, et al. demand, and contractile fibers can neither be demonstrated nor can they be conceived to exist in endoplasm which exhibits all the essential properties of a fluid.
(2)The imbibition theories. Sachs (’65), Hoffmeister (’67)and Englemann (’79) conceived of streaming as being caused by certain constituents of the cell imbibing water and later discharging it. Sachs and Hoffmeister thought that waves of imbibition and extrusion of water passing progressively along the cell was able to cause movement of the protoplasm. Ewart (’03) has shown, however, that as much as 2000 times its own volume of water would have to be imbibed by a cell ofNitellain the course of a day to account for the amount of streaming observed, and that no sign of the extrusion of water could be detected by observing small suspended particles in the immediate vicinity of the cell. Englemann’s theory involving a change of shape of his hypothetical supra-molecular “Inotagmas,” by the imbibition of water and the subsequent release if it, which was supposed to account for the movement of the protoplasm while streaming, has been considered too hypothetical and too far removed from the realm of experiment to be of real value, either as an explanation or as a working hypothesis.
(3)The oxidation theory of Verworn. Verworn (’92, ’09) has postulated a “Biogen Molecule” which exists only in living protoplasm and dissociates when protoplasm dies into a number of chemical molecules of albumin and other substances. Ameboid movement and streaming generally, according to Verworn, is caused by the lowering of the superficial surface tension in the moving mass of protoplasm followed by streaming of the protoplasm toward the point of lowered tension. The lowering of the surface tension is brought about by a union of the Biogen Molecule with oxygen. With the dissociation of the biogen-oxygen compound, presumably through a respiratory process, the surface tension rises again. This theory does not hold for amebas, for we saw in the preceding pages that the surface tension is higher at the anterior ends of pseudopods than elsewhere on the ameba. And in plants, as Ewart (’03) has shown, oxygen does not seem necessary to the streaming process, for the endoplasm ofCharacells continues to stream for many days in the entire absence of oxygen. It is possible that there would be enough loosely fixed oxygen in the endoplasm ofCharato supply the demands of Verworn’s theory; but the very hypothetical nature of his theory prevents one from discussing this possibility.
(4)The electrical theories. These fall into three classes: (a)The galvanic theory. Amici (’18) suggested that the chloroplastids floating in the endoplasm of plant cells acted as galvanic cells, setting up currents in the endoplasm which in some way caused the endoplasm to move. Dutrochet and Becquerel (’38) also held to this explanation. A fatal defect of this theory is that streaming occurs in a great variety of cells, myxomycete plasmodia, amebas, stamen hairs ofTradescantia, etc., in which no chloroplastids occur; and there is no ground for assuming that the causes of streaming in cells with chloroplastids is fundamentally different from that in other cells. (b)The electromagnetic theory. Velten (’72, ’73) and Hörmann (’98) are chiefly responsible for the development of the electromagnetic theory. They hold that chloroplastids have an independent movement of their own; but the principal postulate of this theory is that there is electric repulsion between the ectoplasm and the endoplasm. Ewart (’03) has pointed out, however, that this theory is contradicted by the fact that when streaming becomes very active inElodea, the ectoplasm becomes exceedingly thin and therefore would show movement in the direction opposite to that of the endoplasm if there were magnetic repulsion between these layers. Moreover, the formation of threads of endoplasm across the central vacuoles in plant cells, and the much branched network of pseudopods in plasmodia and foraminifera would be very difficult if not quite impossible to explain on this assumption. (c)The electro-chemical surface-tension theory of Ewart. As the result of a considerable amount of experiment and observation on endoplasmic streaming in plants, Ewart (’03) has come to the conclusion that there are differences in electrical potential between the protoplasm-vacuole boundary and the protoplasm-cell wall boundary, and that as a consequence electrical currents are passing between these points, traversing the protoplasmic stream. If now it is assumed that the particles in the endoplasm, which are electrically polarized, have the surface tension of their corresponding ends decreased when electric currents traverse the endoplasmic stream, the particles and, of necessity, the whole stream of endoplasm would move in the direction of lowered surface tension (Figure 31, p. 96). Continuous chemical actions would benecessary to maintain the conditions as outlined. This theory accords with the facts so far as it goes, but it does not explain the streaming in threads across the vacuole in the plant cell, thus necessitating two theories for the explanation of streaming within a single cell at the same moment. Moreover a central vacuole of cell sap seems always to be required to fulfill the conditions of this theory, and this, as is readily seen, makes it impossible to apply it to streaming in amebas, myxomycetes, foraminifera and ciliates.
The fundamental cause of streaming is therefore still to be discovered, for neither the theories of streaming as applied to ameba, nor those described above which refer especially to plant cells, are satisfactory. But a significant point in these theories is that with increasing information, they come more and more to demand a colloidal structure in the protoplasm. It is the surface energy in the interfaces in the colloidal system which comes to be regarded as the primary source of the energy. But all attempts thus far to explain exactly how this energy is utilized have been unsuccessful. Gaidukov’s (’10) observation is of some interest, however in this connection. He found that the occasional stopping of streaming in cells ofVallisneriais accompanied by a cessation of Brownian movement, which indicates a change from a sol to a gel state. This proves therefore that colloidal changes are possible in streaming protoplasm, and that the general search for an explanation of streaming along this line is proceeding in the right direction. The researches of Bancroft (’13, ’14) and especially of Clowes (’13, ’16) on the nature of the change of phase in emulsions are very instructive in this connection; and it is undoubtedly true that as rapid progress is now being made by the investigation of colloidal solutions as by the direct study of protoplasm, in solving the problem of streaming.
The problem of thecontrolof the streaming process, which is of course much the most important feature of streaming, will probably be solved, at least in part, when the mechanics of streaming is understood.
The discussion of the surface film of ameba and its movements during locomotion naturally led to a discussion of the various theories that have been offered to explain ameboid movement and protoplasmic streaming. Now the fact that the ameba possesses a traveling surface film which can carry particles recalls similar behavior in Oscillatorias and in the diatoms. No new observations have been made very recently, but by comparison of the behavior of particles carried by an Oscillatoria filament and by ameba, it is found that the nature of the movement, the rate of movement, the degree of adhesion of the particles, the sizes of the particles carried and so on, are similar in both organisms. This indicates that there is a surface layer on Oscillatoria threads that is similar to that which has been described in amebas, and whose movement is probably also effected by changes in surface extension; but just how this change is effected is not clear owing to the spiral path the particles take as they travel along the Oscillatoria filament. The spiral has an angle of about sixty degrees, which must be related in some way to the finer structure of the cells of which the filament is composed. The suggestion that movement is caused by the rapid and forcible exudation of mucus is exceedingly improbable if not physically impossible. It is difficult to see how the spiral direction of the flow of mucus could be brought about, to say nothing of the frequent change in direction of the flow. In a surface tension film, however, the direction of movement is readily determined by the location of the points where the tension is changed. Mucus secreting glands would need special structural devices such as secretory tubes bent at an angle to control the direction of flow, while no such structural devices are necessary if the propelling force is surface energy. In short, it is difficult to see how any movement at all could be produced by the act of secretion of mucus, while from what we have seen in the ameba, surface tension changes could easily produce movementin Oscillatoria. The spiral feature of the movement has no explanation that is based on observational data. It may be added here that the surface film in amebas is powerful enough to enable them to move by means of it. One sometimes seessphaeronucleosusor small individuals ofverrucosa, that are lying loose on the substratum, actively streaming, but moving slowly and more or less irregularly backwards. This movement is due to the activity of the surface film.
The suggestion that no extra-cellular protoplasmic layer has been demonstrated in Oscillatoria is not a cogent argument against the surface tension hypothesis, since the surface film would need to be but a small fraction of a micron thick, too thin to be demonstrated by histological methods now in vogue. It is also to be remembered that the surface film in ameba can be demonstrated in no other way at present than by its particle-carrying capacity.
The main features of the movements of diatoms are very similar to those of Oscillatoria. Müller (’89, ’97, ’99) has shown that the gliding movements of diatoms are not due to the ejection of water, but to the streaming of protoplasm on the outside of the shells. Foreign particles are carried by these shallow streams of protoplasm in quite the same manner as by the surface film of the ameba. And there seems to be no evidence against the assumption that these shallow streams, at least the surface films over them, owe their movement to changes in surface tension.
Desmids also glide about slowly, leaving a track of mucus behind. Only one explanation for locomotion has been advanced, and that is that it is due to the secretion of mucus (Klebs, ’85). This explanation is likely to be as wide of the mark as the similar explanation in the case of Oscillatoria. There is no question concerning the excretion of mucus, but the source of the locomotive energy is probably here also surface energy, though the observational data are too few to try to locate the regions where the changes in tension occur.
It has been a matter of considerable surprise to me to find that the so-called “crawling” euglenas, in addition to the diatoms, desmids, Oscillatoria, Beggiatoa and perhaps other forms of life such as the Gregarinidas, also possess extra-cellular films whichcarry particles as do amebas and Oscillatoria, and move about through the agency of this film. The film travels spirally around the euglena as it does in Oscillatoria filaments. In at least two species the film moves parallel to the spiral striations on the outer surface. In one species no spiral striations could be detected, although the film moved spirally. The species of euglenas in which these movements were observed, were not identified.
The character of the movement of the euglenas is very similar to that of the diatoms excepting that most of the diatoms do not revolve on their longitudinal axes. The movement of particles on the surface film of euglenas is quite like that in Oscillatoria, though it is only under exceptional circumstances that one can see particles attached to the surface film. The movement of the particles indicates that the surface film moves from the anterior end toward the posterior end, but whether the “spine” is to be included was not definitely determined. The degree of cohesiveness of the film is high, for locomotion is rapid, even if only a small part of the posterior end is in contact with the substratum, as when moving over an Oscillatoria filament. To one who has seen the movements of the surface films of amebas, diatoms and Oscillatorial filaments, the most reasonable conclusion seems to be that the cause of locomotion in crawling euglenas is the same as that in Oscillatoria and diatoms.
Evidence contributing to this conclusion is found in the circumstance that crawling euglenas, diatoms and Oscillatoria threads are much more refractory to galvanic currents than flagellate euglenas or other flagellates or ciliates: The electrical apparatus at my disposal was rather crude, but I was unable to find that I could influence the direction or character of movement of Oscillatoria filaments, diatoms or crawling euglenas without injuring the organisms. Currents which had produced a marked effect on ciliates or flagellates produced no effect whatever on amebas, diatoms, Oscillatoria or crawling euglenas. Diatoms are particularly resistant to the effect of electrical currents.
The general conclusion regarding the source of energy of the moving surface films, whether found on amebas, diatoms, desmids, or crawling euglenas, is that all derive their motive power from the energy in the superficial films of these organisms; whileameboid streaming, if it is a surface tension phenomenon as seems to be the case, depends upon the surface energy of the interfaces of the emulsoid colloidal system in the endoplasm. It has already been seen that those cases of locomotion due in large measure to the power of contractility in the ectoplasm (Difflugia, Foraminifera) are also explained as being due to a change of phase in the colloidal system, which is in itself a surface tension effect. It appears therefore that all the lower organisms that move, excepting flagellated or ciliated organisms (of whose motor mechanism we have no detailed knowledge), depend upon surface energy as the source of the energy of movement.