PSEUDOPODS AND THE NATURE OF THE ECTOPLASM
In contrast with the ridge-forming amebas stand those with smooth
ectoplasm, such as the common _dubia_, _discoides_, _villosa_, and the
rarer _laureata_ and _annulata_, to mention only a few of the larger
forms. In addition to these may be mentioned all the pelomyxas and
nearly all the smaller amebas. Much the larger number of species of
amebas do not form ridges in the ectoplasm during locomotion.
[Illustration: Figure 4. _Amoeba laureata._ This ameba is multinucleate,
containing a thousand or more nuclei of the shape shown at the right.
Ameba 1000 microns long in locomotion. Nuclei 10 microns in diameter.]
Of all the amebas with smooth surfaces, the most favorable for
observation as to the formation of ectoplasm, is the giant _laureata_
(Figure 4), though it is unfortunately of infrequent occurrence. This
species is as often found in clavate form as with pseudopods. In cross
section it is circular or nearly so. It is often found with
_zoochlorella_ growing in it, upon which it seems to depend largely for
food, for it seldom has distinctive food masses in it. The nuclei are
small and very numerous and the crystals are well formed and numerous,
each in a small vacuole, and of a size about two or three times those
found in _proteus_. It will be seen therefore that there are only small
bodies in this ameba, none of which (excepting the contractile vacuole)
are large enough to change the course of the endoplasmic stream, and
streaming is thus reduced to what might be called a typical condition.
In this ameba the endoplasmic stream flows uniformly towards the
anterior end where it spreads out slightly so as to preserve the same
general diameter of the ameba, for it is a characteristic of this ameba
that the anterior end is of about the same diameter as the posterior,
when in clavate form. The ectoplasmic tube is built at the anterior end,
and remains as constructed until it is drawn in at the posterior end to
form endoplasm. It is not all the time undergoing changes such as are
observed in _proteus_. This characteristic is very well shown by
focusing with the high power of the microscope on the upper surface of
the ameba. The immobility of the ectoplasm is much more readily observed
in _laureata_ than in perhaps any other species, a condition that is due
chiefly to the large crystals whose displacement is the most convenient
criterion of ectoplasmic mobility.
The ectoplasmic tube is not as thick as in _proteus_, 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 endoplasmic stream). 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 in _laureata_ is 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 as _Amoeba
laureata_, that is, in clavate form and more or less cylindrical in
shape. This is especially the case with _Pelomyxa palustris_ and _P.
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 in _laureata_, but sudden and eruptive, indicating a less coherent
ectoplasm.
The nearest approach to the conditions of streaming as found in _Amoeba
laureata_ is found in _A. discoides_ (Figure 11, _B_) a species often
confounded with _proteus_. 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 no _main_ pseudopod as
does _proteus_, _discoides_, _laureata_ and 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.
[Illustration: Figure 5. _Amoeba limicola_, after Penard. Figures _a_,
_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 in _a_, _b_, _e_. _d_, the nucleus found in _f_.]
There is another group of amebas in which the endoplasm is much more
fluid than in _dubia_. To this group belong _Amoeba limicola_ (Figure 5)
and _Pelomyxa schiedti_ (Figure 6). The latter never forms pseudopods,
and the former does so very seldom. _A. limicola_ is 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. The ectoplasm 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.
[Illustration: Figure 6. _Pelomyxa schiedti_, after Schaeffer. _b_,
bacterial rods characteristic of the genus _Pelomyxa_. _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 schiedti_ moves in much the same way that _Amoeba limicola_
does; 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 glycogen bodies (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 from _limicola_ in that a cross section of the
body is very nearly a circle.
[Illustration: 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 at _d_.]
Another very interesting feature of _Pelomyxa schiedti_ is 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 while _schiedti_ is
unable to form pseudopods at its anterior end, it forms uroidal
projections with great 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 small _Amoeba radiosa_
(Figure 7), _A. bigemma_ (Figure 8) and a new species which for
convenience will be referred to as _bilzi_.
[Illustration: 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 stage _a_. _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 of _a_, 150 microns; of _d_, 12 microns;
of _f_, 2 microns.]
It is well known that _radiosa_ has 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 besides _radiosa_
(see p. 128 seq.)
Another species of ameba in which a trophic as well as a rayed stage is
found, is the recently described species _bigemma_. 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 in _proteus_,
_dubia_, or _laureata_. 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 species _Amoeba 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 a _sphaeronucleosus_.
[Illustration: 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 of
_radiosa_, Figure 5, p. 30, somewhat) and the stage shown at _a_. The
whole of the ameba flows into the broad pseudopod with the arrow. Length
of _a_, 90 microns.]
In size it is about midway between the latter species and _striata_. It
always has a number of prominent longitudinal ridges on its upper
surface. Its mode of streaming is essentially like that of _striata_ or
_sphaeronucleosus_. 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 in _radiosa_ and
_bigemma_, are not pseudopods of locomotion but of _position_; 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 in _bigemma_ and _bilzi_, 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 from _limicola_ to _verrucosa_. But from
_limicola_ to _bilzi_ the number of pseudopods directing locomotion
increases from one to an average of about twelve in _dubia_, and then
falls gradually to one in _bilzi_ and the others beyond in the list.
(See Figure 10.) Where the directive pseudopods begin to disappear, the
transitional appear, viz., in _bigemma_ and _bilzi_; 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 in _bigemma_ and _bilzi_ while in _radiosa_ they occur at almost
all times. In _sphaeronucleosus_ and _verrucosa_ no 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 in _proteus_ in 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 may be 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.
[Illustration: 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 of
_limicola_ as 1 and that of _verrucosa_ as 6.]
That the number and character of pseudopods formed depends _in large
part_ upon 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. In _bigemma_, _bilzi_ and _radiosa_, 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 formation and 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_, _dubia_ and _discoides_, 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 division phenomena, 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 with
Figure 11.
[Illustration: Figure 11. _A_, _Amoeba_ proteus in locomotion. Note
especially the longitudinal ridges. _a_^{1}, equatorial view of the
_discoid_ nucleus. _a_^{2}, a polar view of the nucleus. _a_^{3},
equatorial view of a folded or crushed nucleus frequently found in large
individuals. _a_^{4}, shape of crystals found in this species._B_,
_Amoeba discoides_ in locomotion. _b_^{1}, _b_^{2}, equatorial and polar
views of the _discoid_ nucleus. _b_^{3}, shape of the crystals found in
the ameba. _C_, _Amoeba dubia_ in locomotion. _c_^{1} and _c_^{2},
equatorial and polar views of the _ovoid_ nucleus. _c_^{3}-_c_^{10},
shapes of crystals found in _dubia_. 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;
_a_^{1}, 46 × 12; _b_^{1}, 40 × 18; _c_^{1}, 40 × 32; _a_^{4}, maximum,
4.5; _b_^{3}, maximum, 2.5; _c_^{3}-_c_^{10}, 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
| _Amoeba | _Amoeba | _Amoeba
Characteristics | discoides_ | proteus_ | dubia_
------------------+-----------------+------------------+------------------
Size in locomotion|450 microns |600 microns |400 microns
Pseudopods |cylindrical |dorso-ventrally |dorso-ventrally
| | flattened | flattened
|smooth ectoplasm |folded ectoplasm |smooth ectoplasm
|“main” pseudopod |“main” pseudopod |no “main” pseudopod
| present | present |
|cross section |cross section an |cross section
| circular | irregular oval | oval
|average number |average number |average number
| in locomotion, | in locomotion, | in locomotion,
| three | five | twelve
Crystals |very numerous |less than in |relatively few
| | _discoides_ |
|all uniform |all uniform |at least four
|truncated | truncated | varieties present;
|bi-pyramids | bi-pyramids; | few perfect
| | rarely a | crystals
| | few flat plates |
|maximum size |maximum size | maximum size
| 2.5µ 4.5µ 10µ, 12µ, 30µ
Fission |slower than |average 1 division| faster than _proteus_
| _proteus_ | in 48 hours at |
| | 20° C. |
Maximum time | | |
between divisions|20 days |8 days |6 days
Multinuclearity |binucleate |binucleate |binucleate very
| occasionally | frequent; |
| | tetranucleate |rarely
| | occasional |
Nucleus, shape |biconcave disc, |biconcave disc, |ovoidal
| never folded | frequently folded|
size |40µ × 18µ 46µ × 12µ 40µ × 32µ
General resistance| | |
to same | | |
conditions |slight |very great |greater than
| | | _discoides_
Surface of | | |
posterior end |free from debris |free from debris |carries debris
Effect of | | |
mechanical | | |
stimuli |slightly |responsive |very responsive
| responsive | |
Food cups |small |large |often enormous
Reaction to |readily eaten; |readily eaten; |eaten only occasionally;
carmine | rejected in a |rejected in a | often
| few minutes | few minutes | retained for hrs.
Distribution |sporadic, small |very common |sporadic, frequently
| numbers | | in large numbers
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 are _chemically_
different; 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.