CHAPTER VPseudopods and the Nature of the Ectoplasm

Figure 3. Formation of longitudinal ridges and grooves in the ectoplasm ofAmoeba proteus.A,B,C,D, showing stages in the development of a single pseudopod.a,b,c,d,d1, cross sections of pseudopods at the levels indicated. The arrows show the direction of endoplasmic streaming with special reference to the formation of ridges. The numerals 1 to 7 indicate the order in which the ridges were formed. Note the tongues of ectoplasm which extend into the endoplasm, in the cross sections.

sympathetically-minded naturalist, that of the large number of subsequent writers on ameboid movement only one (Penard, ’02, p. 63) seems to have noticed these folds. Leidy says that “ ... the main trunk and larger pseudopods of the same ameba (proteus) assumed more or less the appearance of being longitudinally folded. The endosarc axially flowed as if in the interior of thick walled canals, of which the walls appeared to be composed of finer granular matter with scattered imbedded crystals. In the flow, all the contents did not move with the same rapidity, and usually the smaller particles were swept quickly by the larger ones. Other matter, including some of the largest elements appeared to stick to the inner surface of the extemporaneous tubes, but successively became detached to be carried along with the rest of the contents (p. 46).” “The endosarc appeared to flow within thick walls of ectosarc which often seemed to be longitudinally folded (p. 326).” Penard (’02) confirms Leidy’s observation as to the existence of these folds: “The current (of endoplasm) indeed is not unified, but there exist many currents at the same time because of the fact that the endosarc is divided into acertain number of longitudinal canals or grooves by dense walls, which are of a temporary nature, being broken down and built up from time to time. It is easy to distinguish one canal from the other in this species, the currents being at first more or less parallel, but terminating at the forward end, by their coalescence, as a single mass of liquid (p. 63).” But Penard questions Leidy’s conclusion that the walls are of ectoplasm: “Moreover Leidy deceives himself without any doubt in considering these partitions as folds of theectosarc. The latter, in the rhizopods, is not a special substance, it is a plasma of surface, specialized for the functions which it has to perform, capable of modification as to its intimate structure, but only so temporarily (p. 63).”

Although it is a very simple matter to prove to one’s satisfaction the mere existence of these folds—a few minutes’ observation under the high power of the microscope will do that—it is a much more difficult matter to observe how these folds originate, because of the incessant changes going on, as recorded by Leidy.

Very young or small pseudopods inproteushave the same general appearance as the pseudopods of other large species (dubia,laureata,discoides,annulata, etc.); that is, there is a central axial stream of endoplasm surrounded by a layer of ectoplasm. But there is one difference even here, and that is the greater thickness of the ectoplasmic walls inproteusin proportion to the diameter of the pseudopod. The ectoplasmic tube however is not solid throughout, but is more or less honeycombed, somewhat like a network, with the spaces filled by endoplasm.

If the ectoplasm is actually endoplasm that has passed into the gel state, then the honeycomb condition just described resembles an intermediate stage where only a part of the endoplasm has been transformed. This network of endoplasm is strong enough however to impede the flow of the main stream of endoplasm along the sides of the pseudopod; but when large objects, such as the nucleus or food masses, too large to be readily carried in the endoplasmic stream, impinge against the imperfectly solidified sides of the tube of ectoplasm, the innermost strands of the spongy network of ectoplasm snap, usually with readiness, allowing the large object to pass by.

The surface of a young pseudopod is smooth, a cross sectionbeing oval in shape (Figure 3,a); but as the pseudopod increases in size, large folds or ridges begin to make their appearance. Usually the first ridges to appear are lateral. They begin as small waves of hyaloplasm which flow out along the sides of the pseudopod for a short distance and then continue to move forward. The endoplasm then flows in a number of small parallel streams amid numerous obstructions through the ectoplasmic tube of the pseudopod into the wave of ectoplasm. After the ridge is well begun, there is frequently observed a slow forward-moving stream of endoplasm within it, but the ridge is never closed from the main endoplasmic stream, as is readily proved by the numerous small streams of endoplasm which continually filter through the ectoplasm into the ridge.

In addition to the lateral ridges, which, as stated, are usually formed first, there appear ridges on the upper side of the pseudopod as well, and presumably also on the under side. So far as could be determined these ridges are all formed in much the same way; that is, by the projection of a small wave of protoplasm from some part of the surface of the pseudopod. The ridges do not always grow by extension at the anterior end as described above. Not infrequently a ridge ten to twenty times as long as wide is pushed out along its whole length at once. This is especially likely to happen in a slender pseudopod that suddenly becomes the main pseudopod. The width of a ridge, especially on the upper surface, does not change much after formation. One can frequently find two or three ridges of about the same width, which run the whole length of the ameba with the exception of a short distance at the anterior end, where, as before stated, there are no ridges.

As the figure indicates, new ridges may be formed from previous ones, either by lateral or endwise extension. In such case the walls of the ridge send out thin waves of hyaloplasm followed by streams of endoplasm, as described above in the formation of the first ridge on a pseudopod. When a pseudopod forms a branch, the ridges on the old pseudopod do not likewise branch, but new ridges are formed which have no connection with old ones, but they may later coalesce with old ridges. Such coalescence is however exceptional. Once a ridge is formed, it retainsits identity as a rule; that is, as the ameba moves forward, the ridge in effect moves back over the ameba to lose itself in the wrinkles at the posterior end (SeeFigure 11,A). The number of ridges on any random selection of amebas is variable, and is moreover difficult to state. A large ameba may have as many as six or seven side by side on its upper surface. The number on the sides and on the lower surface are difficult to estimate. The space between ridges is about equal to the width of the ridges, but as one passes toward the posterior end, the ridges become more closely crowded together.

From these observations on the formation of ridges it is evident that they do not represent a wrinkling of the surface such as occurs in a semi-rigid curved surface when it is made to occupy a smaller space. The ridges are wrinkles only in appearance, not in origin. The surface of the ridges is younger than the space between them. It appears as if the pseudopod which has to widen as it increases in length, could not liquify the ectoplasm uniformly all around, but only in longitudinal strips here and there, and that through these openings the ectoplasm then flows. There is no question about the greater readiness with which ectoplasm is formed in this ameba as compared with many others, but after a careful comparison ofproteusandcarolinensis, where ridges are formed, withdiscoides(Figure 11,B),dubia(Figure 11,C),laureata(Figure 4) andannulata, where none are formed, the only conclusion presenting itself is that the visible physical properties of the protoplasm ofproteusandcarolinensisgive no hint as to the cause of the presence of ridges in these species. The protoplasm ofdiscoidesandlaureatais about as viscous as that ofproteus, yet in these there is never any ridge formation.

The ridges inproteusrecall, of course, the ridges always observed inverrucosa,sphaeronucleosus(Figure 13) and their congeners, especially while the latter are in locomotion. Asphaeronucleosusis especially favorable for study in this connection because of its greater activity. This ameba has four or more longitudinal ridges on its upper surface, while in locomotion, which strongly resemble those inproteusandcarolinensis. The chief difference lies in the fact that insphaeronucleosusthe ridges areextended at their anterior ends continually, and unless the direction of locomotion is changed, the ridges may retain their identity while the ameba moves several scores of times the length of its body. Along the sides, however, new ridges are continually replacing older ones. When the direction of locomotion is changed, the old ridges usually all disappear into a jumble of ridges and crinkles running in every conceivable direction, and with the reestablishment of locomotion along a more or less straight path, a new set of ridges appears. Insphaeronucleosusand its congeners, the ridges are also not wrinkles, but ridges that are formed later than the surface contiguous to them.

It is interesting to recall also that the ectoplasm insphaeronucleosus,verrucosaand the rest of this group, is much firmer than in most other amebas.

In contrast with the ridge-forming amebas stand those with smooth ectoplasm, such as the commondubia,discoides,villosa, and the rarerlaureataandannulata, to mention only a few of the larger forms. In addition to these may be mentioned all the pelomyxas and nearly all the smaller amebas. Much the larger number of species of amebas do not form ridges in the ectoplasm during locomotion.

Figure 4.Amoeba laureata.This ameba is multinucleate, containing a thousand or more nuclei of the shape shown at the right. Ameba 1000 microns long in locomotion. Nuclei 10 microns in diameter.

Of all the amebas with smooth surfaces, the most favorable for observation as to the formation of ectoplasm, is the giantlaureata(Figure 4), though it is unfortunately of infrequent occurrence. This species is as often found in clavate form as with pseudopods. In cross section it is circular or nearly so. It is often found withzoochlorellagrowing in it, upon which it seems to depend largely for food, for it seldom has distinctive food masses in it.The nuclei are small and very numerous and the crystals are well formed and numerous, each in a small vacuole, and of a size about two or three times those found inproteus. It will be seen therefore that there are only small bodies in this ameba, none of which (excepting the contractile vacuole) are large enough to change the course of the endoplasmic stream, and streaming is thus reduced to what might be called a typical condition.

In this ameba the endoplasmic stream flows uniformly towards the anterior end where it spreads out slightly so as to preserve the same general diameter of the ameba, for it is a characteristic of this ameba that the anterior end is of about the same diameter as the posterior, when in clavate form. The ectoplasmic tube is built at the anterior end, and remains as constructed until it is drawn in at the posterior end to form endoplasm. It is not all the time undergoing changes such as are observed inproteus. This characteristic is very well shown by focusing with the high power of the microscope on the upper surface of the ameba. The immobility of the ectoplasm is much more readily observed inlaureatathan in perhaps any other species, a condition that is due chiefly to the large crystals whose displacement is the most convenient criterion of ectoplasmic mobility.

The ectoplasmic tube is not as thick as inproteus, though it appears to be more solid than in that species. It is thrown into folds at the posterior end as it is liquified to form endoplasm, which indicates a firm texture of the ectoplasm. As to the endoplasmic stream, it presents no visible characteristics which set it apart from the fluids of physics; it moves most rapidly in the middle, and gradually less rapidly as the ectoplasm is approached. There is no backward movement of the ectoplasm against the sides of the pseudopod at the anterior end—nothing approaching a “fountain current”—which indicates that the transformation of endoplasm into ectoplasm is rapid and complete. That is, all the endoplasm which reaches the anterior end is turned into ectoplasm. Typically this would result in an ameba of average size, in a layer of ectoplasm of a thickness of about one-seventh of the diameter of the pseudopod (for the area of the cut ectoplasmic tube would equal the area of the endoplasmicstream). But because of friction against the sides of the ectoplasmic tube, there is a layer of endoplasm of appreciable thickness that is practically motionless. This layer of endoplasm therefore makes the diameter of the endoplasmic stream appear smaller than it actually is, and the ectoplasmic tube larger than it is. The actual thickness of the tube of ectoplasm, as distinguished from the flowing endoplasm, is difficult to measure, but it seems to be about one-tenth the diameter of the pseudopod. (Kite (’13) found ameboid ectoplasm to be from eight to twelve microns thick, but he does not state from what part of the ameba nor from what species the ectoplasm was taken.) This would indicate that if the transformation of endoplasm into ectoplasm is as complete as the conditions permit, the thickness of the friction layer would be about one-twenty-third of the diameter of the pseudopod. These observations therefore point to the conclusion that the tendency inlaureatais for all the endoplasm to be transformed into ectoplasm at the anterior end, and for the reverse process to occur at the posterior end.

Several of the pelomyxas also move in much the same manner asAmoeba laureata, that is, in clavate form and more or less cylindrical in shape. This is especially the case withPelomyxa palustrisandP. belevskii. But in these species the endoplasm is not completely converted into ectoplasm at the anterior end, as is shown by the fact that there is a slight backward current of endoplasm at the sides near the anterior end (Schultze, ’75). Observation indicates also that the ectoplasmic tube is thinner than would be the case were there complete transformation of endoplasm into ectoplasm at the anterior end. The origin of pseudopods in these pelomyxas is not steady and under control as inlaureata, but sudden and eruptive, indicating a less coherent ectoplasm.

The nearest approach to the conditions of streaming as found inAmoeba laureatais found inA. discoides(Figure 11,B) a species often confounded withproteus. This species is frequently found in clavate form, and the conversion of endoplasm into ectoplasm is complete at the anterior end. In other respects of streaming and pseudopod formation, the two species are also similar.

In another very common species of ameba,Amoeba dubia(Figure 11,C) the clavate stage of locomotion is comparatively rare, but when it is found it is observed that the transformation of endoplasm into ectoplasm at the anterior end is incomplete, and the endoplasm seems to be of very liquid consistency. This ameba is characterized by the possession, usually, of numerous pseudopods extending from a central mass of protoplasm. In this stage it possesses nomainpseudopod as doesproteus,discoides,laureataand other species, but there are three or four pseudopods extending actively in the general direction of locomotion. The physical characteristics of these pseudopods, in so far as streaming is affected, are different from those of the clavate amebas. The ectoplasmic tubes are relatively thicker, the endoplasm is less fluid, and new pseudopods are not formed so readily. It appears therefore that an increase of surface in the ameba serves to increase the amount of ectoplasm that is formed during locomotion.

Figure 5.Amoeba limicola, after Penard. Figuresa,b,e, illustrate the “eruptive pseudopods” by means of which this ameba moves.f, a variety or separate species whose ectoplasm is somewhat firmer, and whose posterior end possesses a conspicuous uroid.c, the nucleus found ina,b,e.d, the nucleus found inf.

There is another group of amebas in which the endoplasm is much more fluid than indubia. To this group belongAmoeba limicola(Figure 5) andPelomyxa schiedti(Figure 6). The latter never forms pseudopods, and the former does so very seldom.A. limicolais extremely fluid, and in locomotion the flow of the endoplasm can hardly be called streaming, for it rushes about in the body as if it were only partially under control. Theectoplasm does not give way steadily at the anterior end during locomotion, allowing a steady forward flow of the endoplasm, but it breaks away suddenly here or there, allowing the endoplasm to rush through as if it were under considerable pressure. When the endoplasm rushes through these breaches in the ectoplasm, it is usually deflected back along the side of the ameba for a considerable distance, thus leaving a part of the old ectoplasmic wall stand for a few seconds between the reflected wave of ectoplasm and the main body of the ameba. It is then that one can observe especially well the very thin ectoplasm covering the ameba, the thickness of which is about one-fortieth the diameter of the ameba. This ameba is somewhat dorso-ventrally flattened and generally oblong in shape during locomotion.

Figure 6.Pelomyxa schiedti, after Schaeffer.b, bacterial rods characteristic of the genusPelomyxa.c, v,contractile vacuole.g, glycogen bodies.n, nucleus.u, uroidal projections. At the left is shown a series of outlines of the animal during locomotion. Length, about 75 microns.

Pelomyxa schiedtimoves in much the same way thatAmoeba limicoladoes; that is, by eruptive waves of endoplasm which are usually deflected back along the side (Figure 6, at the left). The endoplasm is likewise of very thin consistency. The thinness of the ectoplasm and the ease with which it may be ruptured, is very well shown by the fact that the large irregular glycogenbodies (Štolc, ’00) which fill it to capacity, lie so close to the surface that it is frequently impossible to see any protoplasm between them and the exterior. The contractile vacuoles which are numerous, also testify in their characteristics, to the ease with which the ectoplasm may be broken. The vacuoles never reach but a very small size (four microns in diameter) presumably because of the thin consistency of the endoplasm and because they can readily break through the ectoplasm. They burst on the surface of the ameba instantaneously, as a small air bubble might burst on pure water. But this ameba differs fromlimicolain that a cross section of the body is very nearly a circle.

Figure 7.Amoeba radiosa, after Penard.a, the rayed stage.b, the rayed stage in which some of the pseudopods are being withdrawn. One of them is thrown into a spiral as it is being withdrawn.c, the stage preceding the trophic stage shown atd.

Another very interesting feature ofPelomyxa schiedtiis the uroid (Figure 6,u), which in this species consists of a number of very thin projections resembling pseudopods extending from the posterior end. These projections are attached to the substratum and in some way aid in locomotion. These uroidal projections are of considerable length, and may persist for a considerable length of time. Thus whileschiedtiis unable to form pseudopods at its anterior end, it forms uroidal projections withgreat ease at its posterior end. But what the conditions are which are necessary for the formation of a uroid, a structure which it may be added, exists in many species of amebas (and perhaps also in Cercomonas), is quite unknown.

In contrast to the amebas thus far discussed from the point of view of the transformation of endoplasm into ectoplasm, there are a number of species in which two distinct methods of endoplasmic transformation occur typically. Among these species are the smallAmoeba radiosa(Figure 7),A. bigemma(Figure 8) and a new species which for convenience will be referred to asbilzi.

Figure 8.Amoeba bigemma, after Schaeffer.a, usual form in locomotion, showing the numerous pseudopods, vacuoles, nucleus and food body.b, rayed stage frequently assumed when suspended in the water. The pseudopods in this stage are clear, slender, and more rigid than those in stagea.c, an excretion sphere attached to a twin-crystal characteristic of this ameba.d, the nucleus, consisting of a clear nuclear membrane and a mass of chromatin granules in the center.e, a small sphere attached to a crystal.f, a twin crystal unattached to a sphere. Length ofa, 150 microns; ofd, 12 microns; off, 2 microns.

It is well known thatradiosahas two stages: a more or less clavate shaped stage in which the ameba creeps along the surface of some object (Figure 7,d); and a stage in which a number(eight or less) of long and very slender tapering pseudopods are formed which usually persist for a long time (Figure 7,a,b). These pseudopods are frequently quite straight and regularly disposed around the central mass of protoplasm (Penard, ’02, pp. 87, 89). In no case are any endoplasmic granules found in these slender pseudopods; they consist entirely of hyaloplasm. In retracting these pseudopods a curious phenomenon is sometimes observed; the pseudopod is rolled up into several (as many as six) turns of an almost perfect helical spiral of a diameter six to eight times that of the pseudopod. But as the process of withdrawal proceeds, the spiral becomes irregular, but parts of some of the turns persist in the last vestiges preceding complete withdrawal (Figure 7,b). These spirals are also observed in other species besidesradiosa(see p. 128 seq.)

Another species of ameba in which a trophic as well as a rayed stage is found, is the recently described speciesbigemma. In this species the rayed stage is only of occasional occurrence (Figure 8,b). The larger the ameba is, the rarer is the rayed stage assumed. On very rare occasions one finds a rayed stage in which the pseudopods are long, straight, slender and tapering, and more or less regularly disposed around the central mass of protoplasm. The trophic stage (Figure 8,a) is much the more common. In this condition pseudopods are formed in large number. They are small, conical or linear, and blunt, and they do not determine the direction of locomotion, as they do inproteus,dubia, orlaureata. These pseudopods are often composed only of hyaloplasm, though frequently the basal parts of them consist of endoplasm. When these amebas become suspended in the water, they frequently assume a shape that approaches the rayed condition: six or more long conical pseudopods are run out from the central mass of protoplasm, but the pseudopods are not straight in this case, but irregularly curved and capable of being waved about to a slight extent. The ameba readily passes from this stage to the trophic.

The speciesAmoeba bilzi(Figure 9) has come under my observation on several occasions, and its pseudopodial characters are of considerable interest in this connection. In its usual form this ameba has the general appearance of asphaeronucleosus.

Figure 9.Amoeba bilzi.a, the ameba in locomotion, showing the ectoplasmic ridges, nucleus, contractile vacuole.b, the transition stage between the rayed stage (which resembles that ofradiosa,Figure 5, p. 30, somewhat) and the stage shown ata. The whole of the ameba flows into the broad pseudopod with the arrow. Length ofa, 90 microns.

In size it is about midway between the latter species andstriata. It always has a number of prominent longitudinal ridges on its upper surface. Its mode of streaming is essentially like that ofstriataorsphaeronucleosus. When this ameba is disturbed and left suspended in the water, it throws out four or five or more long slender pseudopods composed entirely of hyaloplasm, excepting a bulbous base which consists of granular endoplasm. The pseudopods are cylindrical with tapering ends. They are very rigid, and once formed, persist for a considerable length of time. When these pseudopods are about to be retracted, the wall weakens at some point and then crinkles while the distal part of the pseudopod bends, often at a decided angle. The crinkling of the wall continues up and down the pseudopod while it is slowly being withdrawn. These pseudopods, as well as those of the rayed state inradiosaandbigemma, are not pseudopods oflocomotion but ofposition; they are not dynamic but static structures. But there are no hard and fast distinctions to be made between these two types of pseudopods, for at least inbigemmaandbilzi, there are transitional forms of pseudopods (Figure 8,b).

The formation of pseudopods and their character depends to some extent upon the firmness and thickness of the ectoplasmic layer; and the character of the ectoplasm in turn depends largely upon the consistency of the protoplasm as a whole. In the following representative list of amebas:limicola,villosa,dubia,proteus,discoides,laureata,bigemma,bilzi,radiosa,sphaeronucleosus,verrucosa, the given order indicates a progressively thicker and firmer ectoplasm as one passes fromlimicolatoverrucosa. But fromlimicolatobilzithe number of pseudopods directing locomotion increases from one to an average of about twelve indubia, and then falls gradually to one inbilziand the others beyond in the list. (SeeFigure 10.) Where the directive pseudopods begin to disappear, the transitional appear, viz., inbigemmaandbilzi; but beyond these no transitional pseudopods occur. But along with the transitional there begin to appear also the static pseudopods, which are seen relatively seldom inbigemmaandbilziwhile inradiosathey occur at almost all times. Insphaeronucleosusandverrucosano distinctive pseudopods of any kind occur.

If all the known species of amebas in which the necessary characteristics have been recorded, were arranged similarly with respect to the firmness and the thickness of the ectoplasm, the general relations of the various kinds of pseudopods in the list would be approximately the same as in the list given above; but there would appear an exception here and there, indicating the operation of special factors. Such an exception, for example, is seen inproteusin the list of species given, which because of the ridges that it forms (Figure 3) has a smaller number of pseudopods than would be the case if no ridges were formed[2]. It maybe concluded, then, that the number and character of pseudopods depends in large part upon the ectoplasm-forming capacity of the ameba; and that this property is intimately associated with the degree of fluidity of the whole mass of protoplasm in the ameba.

Figure 10. Graph representing the relation of firmness and thickness of the ectoplasm with the number and character of the pseudopods in different species of amebas.a, the average maximum number of pseudopods directing locomotion in the different species of amebas.b, the number of transitional pseudopods.c, the number of static pseudopods.d, the estimated degree of firmness and thickness of the ectoplasm of the various species of amebas, grading that oflimicolaas 1 and that ofverrucosaas 6.

That the number and character of pseudopods formed dependsin large partupon the firmness and thickness of the ectoplasm was said advisedly. For observations indicate that there are other factors which influence the character of pseudopods besides those which also control the formation of ectoplasm. These other factors indicate their presence readily in the details of structure of the pseudopods. Thus the number of directive, transitional or static pseudopods may be the same in two particular species, yet in their intimate structure and appearance they are always found to differ. Inbigemma,bilziandradiosa, for example, the number of static pseudopods when formed is about the same in the three species, but the similarity ends there. For these species differ in the frequency with which pseudopods are formed, in their persistence when once formed, in the ratio of length to average diameter, in the general shape, in the frequency with which straight pseudopods are formed, in the speed of their formationand withdrawal, in the manner of their withdrawal, in their disposition with respect to geometrical pattern, in the character of the bases of the pseudopods, in the form of the free ends, and so on. Many of these characteristics are still further analyzable into numerous other and more detailed characters. And what is true of the static pseudopods is likewise true of the transitional and the directive. Pseudopod formation is however only a small part of the activity of an ameba. The formation of uroidal projections, of vacuoles of various sorts, of crystals, and so on, are some other general activities that are fully as subject to specific variation as pseudopod formation. Again in behavior to food and various other stimuli, in resistance to various factors in the environment, in reproductive processes, and so forth, there is found similar specific peculiarity. In fact, one looks in vain for similarity between any two species of amebas except in their most generalized characters. From my own experience in extended observation of several dozen species, which included a large number of characters, as pointed out above, I have not found two species of which I can confidently assert that any particular character defined as accurately as possible was present in both. In different words, my experience indicates that no two species are alike in any respect whatsoever. Each species appears unique from every point of view and in the smallest definable detail. The concept of specificity therefore is much more fundamental in amebas than has been believed to be the case hitherto (cf. Calkins, ’12). The intimate structure of amebas is indeed similar to that of higher animals where the precipitin reactions (Richet, ’02, ’12; Reichert and Brown, ’09; Dale, ’12; Nuttal, ’04; also Todd, ’14) have indicated that the various albumins are of specific structure and reaction.

As an example of these specific differences, reference may be made to the three species,protus,dubiaanddiscoides, which have been referred to in the past, almost without exception, by the most experienced teachers of biology, as being one species:proteus. Some investigators of ameboid phenomena have likewise confused these different amebas. Below is given a list of some of the most striking characteristics of these three amebas. This list is of course very sketchy. If the nuclear divisionphenomena, for example, were well known, which they are not, those character differences alone would doubtless make a list several times as long as this one. Compare withFigure 11.

Figure 11.A,Amoebaproteus in locomotion. Note especially the longitudinal ridges.a1, equatorial view of thediscoidnucleus.a2, a polar view of the nucleus.a3, equatorial view of a folded or crushed nucleus frequently found in large individuals.a4, shape of crystals found in this species.B,Amoeba discoidesin locomotion.b1,b2, equatorial and polar views of thediscoidnucleus.b3, shape of the crystals found in the ameba.C,Amoeba dubiain locomotion.c1andc2, equatorial and polar views of theovoidnucleus.c3-c10, shapes of crystals found indubia. In these drawings only such characters as are of special interest for the purpose of this work are emphasized. Dimensions in microns:A, 600;B, 450;C, 400;a1, 46 × 12;b1, 40 × 18;c1, 40 × 32;a4, maximum, 4.5;b3, maximum, 2.5;c3-c10, maxima, 10 to 30.

This fundamental uniqueness of all the characters of the various species of amebas naturally gives rise to the question as to what is the cause of this condition of affairs. Why and how

are the different species of amebas so absolutely different, even to the smallest detail? Why are the apparent resemblances and similarities of their more generalized kinetic characters, such as the formation of pseudopods, of ectoplasm, of crystals, of contractile vacuoles, the general character of endoplasmic streaming,the formation of ectoplasmic ridges, and so forth, found, upon analysis, to resolve themselves into a large number of details which differ more strikingly, the corresponding characters of one from those of the other, than do the generalized characters of which they are composed?

These questions apply, of course, to all other organisms as well as to amebas. Unfortunately, however, these questions are at present unanswerable for all organisms. But for the amebas, at least, the problem of form can be rid of some irrelevant matter which, in numerous instances in the past, has been assumed to be properly included.

In the first place, changing a single character of the protoplasm, such as the degree of viscosity, cannot explain the observed diversity of detail; neither can a variation of a number of the physical characters of fluids produce such differences as are observed in the dynamics of the different species of amebas. Our whole experience with the fluids of physics speaks against such an explanation. But, on the other hand, the invisible details of structure of a fluid may become strikingly manifest under certain conditions, namely, those surrounding the process of crystallization. A slight change in the physical condition may produce a considerable variety of crystal shapes, but this variety of shape has nevertheless very definite limits which cannot be overstepped.

Amebas like crystals are also most rigidly and definitely restricted to a certain range of shape, which must be a direct result of the structure of the protoplasm composing them. Amebas in fact are not any more “shapeless” than crystals are; and it would be quite as exact to say that the crystals of water are shapeless since a great variety of shapes are met with in snow, hoar-frost, etc. The fact that corresponding parts of two species of amebas resemble each other less and less closely as they are analyzed into smaller and smaller details, is in itself conclusive evidence that the protoplasms of the amebas arechemicallydifferent; the resemblance between the gross anatomy and physiology between two different species is due to the greater conspicuousness of such characters as are the result of the action of physical processes. That is to say, chemically or molecularly different masses of matter may resemble each other in their molar aspects.

It is to be noted however that the more intimate structure of streaming protoplasm cannot always express itself externally as it can in ameba. As was suggested in the introduction, there is no good reason for supposing that the causes of streaming in the various organisms in which it is observed are fundamentally different. The problem of ameboid movement cannot be considered apart from the streaming of protoplasm in foraminifera, myxomycetes, plant cells, lymphocytes, desmids, diatoms and ciliates. The streaming of endoplasm in some cells, such as in ciliates and plant cells, does not give rise to change of shape of the cell as it does in ameba. In these cases the character of streaming is highly restricted; the unyielding ectoplasm or cell wall as the case may be, prevents any but the most essential features of streaming from occurring. Recalling the analogy of crystallization, streaming in a plant cell or in a ciliate is analogous to crystallization occurring in a tube or vessel too small for the crystals to form properly.

This discussion anent the fundamental chemical uniqueness of each species of ameba is of course not complete without an examination of the views expressed to the contrary. And it is to this side of the discussion that we may now briefly direct our attention.

After the discovery of the ameba by Rösel v. Rosenhof and the introduction of the Linnean system of nomenclature, the number of new species of amebas that were reported increased rapidly. But in 1856 Carter suggested that what had been described asA. radiosaprobably was a young stage ofA. proteus. With the general acceptance of the Darwinian Natural Selection Hypothesis, the ameba came to be looked upon as standing at the bottom of the scale of organisms, and consequently was supposed to lack generally such characters as the higher forms possessed. The ameba became the representative of the “primordial slime” from which by slow stages the other organisms were evolved. So of the sixty odd species which had been described up to Leidy’s (’79) time, Leidy, following the suggestion of Carter, was inclined to think that the great majority of these represented only changes of shape of about four species (not including the several species that were then known to be parasitic). Since Leidy’s time the prevailing tendency has been to regard most of the “new” species as mere environmental or life cycle stages of a very few species. A very noted exception to this tendency, however, has been Penard’s (’02) great work on the amebas and other rhizopods of the Leman Basin, in which he describes forty-five species of amebas (includingGloidium,Protamoeba,Amoeba,Dinamoeba,Pelomyxa), paying attention mainly to the readily observed ectoplasmic and endoplasmic characters, and the appearance of the resting nucleus.

The remarkable discoveries of Vahlkampf (’05) of the nuclear changes during the division process turned the attention of numerous investigators to this field, and the ectoplasmic and endoplasmic characters thenceforward received scant attention. Thus Calkins (’04) came to suggest as Carter had done many years before, thatA. radiosawas merely a young form ofA. proteus. And Doflein (’07) intimated that the protoplasmic characters ofvespertiliocannot be distinguished from those ofverrucosa,radiosa,polypodia,limaxandguttula. Schepotieff (’10) in a similar vein, writes: “Wir werden demnach so bekannte und so lange Zeit als selbstständige und typische Amöbenarten aufgefasste Formen wieA. limax,A. polypodia, undA. radiosanur als Umwandlungsstadien andrer Arten bezeichnen dürfen.” Gläser (’12) remarks: “The most reliable criterion for the classification of the amebas is the division of the nucleus.” Calkins (’12) takes the same view on this point and states that in his opinion the ectoplasmic and endoplasmic characters of amebas conform to four “types,” viz.,proteus,verrucosa,vespertilioandlimax. The enormous amount of work that has been done on the nuclear division changes as compared with the small amount of work on the cytoplasmic structure has thus naturally tended to an over-estimation of the significance of the nuclear changes.

There are objections to making the nuclear changes the basis of the classification of the amebas.

1. In the first place, to classify the amebas means not only labeling the different species accurately, but also to assign to them their proper place in the system of organisms. All organisms are classified with this purpose in view. This is what is meant by anatural system ofclassification as contrasted with anartificial systembased on only a part, arbitrarily selected, of each of the organisms concerned. In the past all artificial systems have been discarded. It is perhaps unnecessary to say that a classification based on nuclear characters would be a highly artificial system. For in no group of organisms has it been found possible thus far to use the nuclear changes as a basis of classification. The great amount of labor that has been expended by cytologists within recent years on the behavior of chromosomes, and the immense amount of work done by the students of genetics, has failed to show any specific relation whatever between the external characters of organisms and the nuclear behavior.[3]Inother words, the peculiarities of mitotic processes have not been found to be correlated with characters in the somatoplasm. It is to be remembered however that all living organisms, with the exception of some of the bacteria, are classified with respect to their external characters, and that in almost all organisms the number of visible and demonstrable specific characters becomes rapidly greater as ontogenetic development proceeds.

2. There is considerable disagreement among the investigators of the nuclear phenomena of amebas as to the actual events occurring during the division process. Cf. Dobell (’14) and Hartmann (’14)in re Amoeba lacertae; Nägler (’09), Gläser (’12) and Wilson (’16) on the presence of a centriole in amebas; etc. The work of Schardinger (’99), Wherry (’13) and Wilson (’16) on the nuclear stages of amebas was done with care, yet Wilson (’16) still remains in doubt as to whether or not these investigators all worked on the same species.

3. Awerinzew (’04, ’06) found that the nuclear changes inAmoeba proteusare similar to those in the heliozoanActinosphaerium; there being thus greater correspondence in the nuclear changes between species belonging to different orders than there is between species in the same genus. Logically thereforeActinosphaeriumwould have to be placed in the same genus withAmoeba proteus.

4. There is the great practical objection that in many of the larger species it is extremely difficult to find suitable division stages even though thousands of individuals are at hand, and the search is continued for days and weeks by an experienced investigator (Dobell, ’10). Experimental work, which is usually done with one of the larger species, would thus be greatly handicapped because of the great difficulty in determining the nature of the organism employed.

From these considerations it appears that the attempt to classify the amebas on the basis of the nuclear changes is highly artificial and exceptional, and if we may judge from past attempts to classify organisms on the basis of a single character, is foredoomed to failure. This conclusion does not apply, however, to very minute amebas in which no specific cytoplasmic characters have yet been established, chiefly because of their very minuteness;such amebas could be given specific names for reference but they could not be classified in a natural system excepting perhaps as a group.

But the definiteness and the consistency with which the nuclear division stages occur in any given species of ameba, lends support to the probability that in these animals the relation existing between the chromatin and the cytoplasm are similar to those observed in higher animals; and that the laws governing the transmission of cytoplasmic characters in amebas are quite as inflexible as those governing somatoplasmic characters in the higher organisms. Among the investigators of cytologic and genetic phenomena (among the multicellulars) the belief is practically unanimous that the elaborate mechanism involved in nuclear division is primarily a design for distributing the factors concerned in heredity. Now it would be very strange indeed if a similar and quite as complicated a mechanism in ameba had no function to perform. For what would be the purpose of the complicated nuclear changes in ameba if not concerned with heredity? As has already been seen, however, there are numerous cytoplasmic characters, in the larger amebas at least, that are inherited from one generation to the next with as little variation as is observed in other organisms (Schaeffer, ’16). The recent work of Jennings (’16) onDifflugiaand Hegner (’18) onArcellaalso indicates that the general processes of inheritance in these organisms which are closely related to amebas, are similar to those observed in higher forms. The conclusion seems justified therefore that the nuclear changes in amebas mean essentially the same thing as in other organisms.

We are now therefore in a position to say that amebas are definitely and thoroughly organized; that they are not really “shapeless”; that they are not more subject to variation than a higher organism is; and that each species differs from all others in probably every visible detail. The large variety of pseudopods observed in different species are seen not to be the result of physical or extrinsic chemical forces acting upon ectoplasms differing in some mere physical character as viscosity. But all these peculiarities are hereditary, and are due to a fundamental chemical structure of the protoplasm which is specific for the species. Thehighly characteristic nature of the pseudopods formed by the amebas of any species, it is seen, is to be referred to the fundamental structure of the protoplasm, probably its stereochemical structure. And what is of especial importance for this discussion, the character of streaming concerned with pseudopod formation and with movement in general, which is specific for each species, is likewise found, to some extent at least, to be conditioned by the specific structure of the protoplasm.

That the specific character of the pseudopods, and the streaming which of course lies back of it, is not wholly or perhaps even largely, due to the specific structure of the protoplasm, is evident from a consideration of streaming in some other organisms, without a study of which, streaming in amebas can be only imperfectly understood.

The formation of pseudopods is not necessary to streaming. Occasionally one sees internal currents unaccompanied by movement or ectoplasm formation in amebas approximating spherical shape, such as inAmoeba blattae(Rhumbler, ’98) and rarely also inproteusordubia. But especially well is such streaming seen in a contractedBiomyxa, a naked foraminifer, and in numerous plant cells. In paramecium and other ciliates the continuous circulation of the endoplasm,—a true streaming process,—is an involuntary act. But inFrontonia, another large ciliate, the circulation of the endoplasm is under the control of the animal, that is to say, voluntary, and is set in motion only when feeding, the direction of streaming being away from the mouth so as to drag in the food (seeFigure 32, p. 99). If the food particle is a long filament ofOscillatoria, for example, the endoplasm circulates very much as it does in paramecium, only more rapidly, until the whole filament is wound up into a coil. Then streaming stops. In the second place streaming is not necessarily accompanied by the formation of ectoplasm as observed in ameba. In plant cells the ectoplasm is practically stationary, while the endoplasm is in continual flux. The transformation of endoplasm into ectoplasm and vice versa is therefore not an essential feature of streaming, though it is of locomotion; that is, ectoplasm is always found between endoplasm and water, though it might be possible under certain conditions for endoplasm to come into contactwith water without stiffening. And if so, there appears to be no reason why locomotion might not occur. It appears however under normal conditions that a moderate tendency to ectoplasm formation (proteus, dubia) leads to greater efficiency in movement than a very weak (limicola) or a very strong (verrucosa) tendency to form ectoplasm.

In the reticulose rhizopods, as is well known there is no ectoplasm of the kind observed in amebas. The middle of the pseudopod, moreover, is not the region of most rapid streaming as in ameba, but frequently becomes congealed, on the contrary, into a rod-like structure. In general this axial rod has the character of very stiff ectoplasm. The character of streaming in reticulose rhizopods, however, has received very little attention, and detailed comparisons are therefore impossible.

Another interesting property of reticulose pseudopods, which are formed by a streaming process, is their great power in some species, of rapid contraction. If a diatom for example, in its movements breaks loose a pseudopod it is often (though not necessarily) contracted very rapidly, much more rapidly than could be the case if it were accomplished by streaming. It frequently happens that knobs are found on a slender pseudopod. These knobs may move back and forth with great rapidity without visibly affecting the pseudopod (Figure 12). The process reminds one of a block sliding on a rope. These observations indicate a very high degree of elasticity in the formed pseudopods of such a rhizopod asBiomyxaas compared with a very low degree of elasticity in the amebas.

It thus appears that the process of streaming is a much more fundamental phenomenon than most of the theories accounting for ameboid movement would lead one to suppose; for these theories concern themselves only with streaming as observed in amebas, and many content themselves with only two or three species. Since the general features of streaming are similar no matter where streaming occurs, no theory is likely to gain acceptance that explains streaming only in one group of organisms. Streaming in rhizopods, myxomycetes, ciliates, plant cells, is most rationally looked upon as caused by the same fundamental process; but the detailed form it takes, especially in freely formedpseudopods, is undoubtedly conditioned by the structure of the protoplasm, both physically and chemically, but more especially the latter.

Back to Index Next