CONCLUSIONS
One of the most important results of recent work on the movements of
ameba and of streaming endoplasm in plant cells is the rapidly growing
conviction that the streaming of protoplasm, wherever it is found, is
due to the same fundamental cause. The value of this conception lies in
the greatly widened front that is presented for attacking the general
problem of streaming. The many special aspects of streaming, which in
the past have been thought to be essential or fundamental processes, may
thus be placed against each other, following what is known as the
comparative method, and the main problem will thus be freed of much that
is not strictly relevant. In this way we come at once to the heart of
the problem.
One of these special aspects of streaming in amebas is the formation of
ectoplasm. For ectoplasm formation is not essential to streaming. But it
is almost certainly essential to locomotion, for locomotion has not been
observed in amebas where ectoplasm was not formed. But, on the other
hand, ectoplasm, as known in the amebas, is not formed without
streaming, although observations indicate that ectoplasm may suddenly
and temporarily pass into the gel state (_Vallisneria_). Streaming is
therefore the fundamental process in ameboid locomotion.
The surface layer of the ameba is physiologically distinct from the
ectoplasm, although it differs from ectoplasm chiefly, if not wholly, by
virtue of its position only. That is, the surface layer is a true
surface tension film. There are no observations recorded which actually
show that the surface film of the ameba is a semi-permeable or plasma
membrane; but, on the other hand, there are no observations which speak
against such a supposition. On theoretical grounds the conclusion is
justifiable that the surface film as demonstrated by the movements of
attached particles is the plasma membrane.
The similarity of the movements of the surface film in ameba with the
movements of the superficial films of _Oscillatoria_ filaments, diatoms,
crawling euglenas, and probably also Gregarinidas, indicates that the
superficial films of all these organisms, including amebas, are all
activated by surface tension changes. Thus instead of postulating
several methods of locomotion which are fundamentally different from
each other, for these respective organisms (excepting the ameba), one
explanation serves the purpose; and it has the further merit of agreeing
more nearly with observation than the various other theories proposed.
From the point of view of ameboid movement, the discovery of the surface
film and its activities narrows down the problem very considerably. It
does not help _directly_ perhaps, in the solution of ameboid movement,
but it shows clearly that the region where ectoplasm is most rapidly
formed (at the anterior ends of pseudopods) is also the region where the
superficial tension is increased. This therefore gives us somewhat of an
insight into what must take place during the transformation of endoplasm
into ectoplasm.
Although the wavy path of the ameba does not at present relate itself to
any other process in the ameba, it is bound to be of the greatest
significance in investigating the intimate nature of protoplasm while in
movement. In so far as the wavy path concerns the ameba, it effectively
disproves the presence of that scientific monstrosity, random movement.
The path of the ameba is orderly.
The wavy path of the ameba represents a projection on a plane surface of
a helical spiral. The path of the ameba is thus geometrically related to
the spiral paths of free-swimming organisms such as ciliates,
flagellates, rotifers, swarm spores, worm larvae, etc. But the paths are
more closely related than merely geometrically. The effects produced by
temperature on amebas and ciliates and flagellates indicate a
relationship between the physical processes underlying the control of
the direction of the paths traveled over in free movement. No causal
distinction can yet be made between rotation on the long axis and the
spiral swinging.
The spiral path is not an acquired habit. It is not a habit that has
been developed to overcome asymmetry of body shape, for some spirally
swimming organisms are not asymmetrical enough to make swimming in
spirals necessary. It is also unlikely that so many thousands of species
of animals and plants of widely different groups would hit upon the same
complex habit to solve widely different problems; for it is not equally
important that all animals should swim in straight paths. It also
necessitates supposing that the ancestors of our present ciliates,
flagellates, rotifers, swarm spores, zoöspores, etc., were symmetrical
and swam without revolving on the long axis and without forming spirals.
Such an assumption is too formidable and makes the explanation
top-heavy.
Spiral swimming is supposed to be due to an automatic regulating
mechanism which is present in all moving organisms. It is held to be a
spatial aspect of the physical processes originating and controlling
movement. The property of moving automatically in an orderly path is
inherent in organisms in the same way, e.g., as the property of growth
is. A spiral path will be followed whenever an organism is free to move,
that is, when not disturbed by sensory stimulation. Slight stimulation
is often without effect. The justification of supposing that probably
all moving organisms are within the grip of the spiral urge is found in
the fact that the amebas, ciliates, flagellates, swarm spores,
zoöspores, Oscillatoria, diatoms, rotifers, larvae of worms, molluscs
and echinoderms, oligochaets, copepods, as well as man, all move in
regular smooth spirals of one kind or another when free from strong
stimulation, and that no organism that is free to move as these are,
moves in a straight or irregular path.
The observations indicate that the same type of mechanism that controls
the direction of the path of an organism also unifies and coördinates
the streaming of the protoplasm of the ameba, the action of the cilia of
the paramecium, or the contraction of the muscles of man, as the case
may be. Why the automatic mechanism controlling the direction of
movement should produce a helical spiral in paramecium, a wavy path or
flattened spiral in ameba, and a series of spirals in man, is not yet
subject to profitable discussion, except of course to point out that
paramecium is not restricted to two dimensions of space as is ameba and
man. In the nature of the case there can be no question but that the
mechanism is one that attaches to the fundamental structure of
protoplasm rather than to the gross morphology. As a mathematical
question, however, the circles occurring in the path of an ameba in low
temperature may serve to connect up the flattened spiral path of the
ameba under optimum conditions with the circular path often observed in
man.
The movement of the ameba thus becomes related to crawling euglenas,
Oscillatoria filaments, diatoms, and perhaps Gregarinidas, because of
the movements of its surface layer; to leucocytes, streaming protoplasm
in the higher plant cells, etc., because of its streaming endoplasm; and
to the locomotory movements of all organisms because of the wavy
character of its path, which betrays the activity of an automatic
regulating mechanism, a type of which is held to be present in every
moving organism.
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FOOTNOTES:
[1] Wilson (’00) describes it as _Pelomyxa_, but it has much closer
affinities with _Amoeba_. It is in fact perhaps the closest relative
of _Amoeba proteus_. Ectoplasm formation, and especially the formation
of ectoplasmic ridges in _carolinensis_, is exactly like that in
_proteus_.
[2] This is shown by the fact that after this ameba has taken on a
spherical shape due to some disturbance in the water, the number of
small ridgeless pseudopods thrown out upon resuming movement, is about
the same as in _dubia_; but after ridges begin to form, the number of
pseudopods decreases.
[3] That is, resemblances in nuclear division stages are not
correlated with corresponding degrees of resemblance in somatic
characters. It is not generally held that the shape or size or number
of chromosomes is correlated with any external characters. It is
the presence of hypothetical factors or genes which are held to be
correlated with somatic characters and their number or arrangement in
a chromosome is not in any way related to their character.
[4] It is possible that Gruber was led to suggest a gelatinous
composition for the layer in question on the strength of assertions
made by several writers that amebas secrete mucus. It is true that
amebas may be displaced by threads of mucus hanging to glass needles
which has collected on the needles while manipulating the amebas in
the culture medium, but that is not to be taken as evidence that the
mucus is secreted by the amebas. Ameba cultures are always full of
gelatinous material formed by bacteria. I have not thus far been able
to convince myself that amebas actually secrete mucus.
[5] According to Ewart (’03) the viscosity of streaming protoplasm
in plant cells lies between η = .04 and η = .2. But the velocity of
streaming endoplasm in ameba is considerably slower than that in
the plant cells which formed the basis for Ewart’s calculations. In
comparison, we may estimate the viscosity of the endoplasm of ameba
as η = .1 dynes per sq. cm. The velocity of streaming endoplasm, as
ascertained by observations on _Amoeba dubia_ (in which the endoplasm
flows usually rapidly)
1
is---- cm. per second.
880
Now, given a unit mass of endoplasm moving at a given instant with a 1
velocity of---- cm. per second against viscosity of η = .1 dynes per
sq. 880 cm., how far will the unit mass travel before coming to rest?
Velocity
Force = Mass × Acceleration, and Acceleration = ---------.
Time
MV
η = Viscosity = F = MA = ----.
T
V
Now if M = 1, η = F = A = ----.
T
1
V ---- 1
T = ---- = 880 = ----.
η ---- 88
.1
The space travelled over in uniformly accelerated motion equals
1 1 1 1 1
S = ---- AT^{2} = ---- × ---- × (----)^{2} = ------ = .00000645 cm.
2 2 10 88 154880
If, therefore, the force moving the central stream of endoplasm should
suddenly be discontinued, the resistance offered by the viscosity of
the enveloping endoplasm would allow it to move only .0000645 mm.
before coming to rest. But the ameba as a whole moves more slowly than
the central stream of endoplasm, the average rate of movement being
about
1
----- mm. per second. The effect of the streaming endoplasm on the
300
forward movement of the whole ameba would therefore be correspondingly
decreased. Now if the ameba was perfectly homogeneous and perfectly
symmetrical, and free from external stimulation, and moved in a
perfectly homogeneous liquid on a perfectly plane surface, the
excessively small amount of mechanical inertia would then be
sufficient, theoretically, to cause the ameba to move in a straight
instead of an irregular path. But these conditions are never realized.
The ameba is unsymmetrical in form, heterogeneous in composition and
always unsymmetrically stimulated; hence it is impossible that the
excessively small amount of mechanical inertia can be considered a
factor in determining the direction of the ameba’s path.
[6] The gap between the rate of movement of a pseudopod and that of
a flagellum is however very wide. Insofar as the _character_ of the
movement is concerned, pseudopods such as those of _flagellipodia_,
probably resemble the flagella of the soil ameba and of flagellates.
But the very much greater speed of contraction of a flagellum and
the presence of a special organ (blepharoplast) at the base of the
flagellum, and their connection with the nucleus, indicates that a
special mechanism is necessary to cause the rapid contraction. A
flagellum appears to be a pseudopod supplied with something like nerve
tissue and a ganglion capable of setting free a rapid succession of
impulses.
[7] It should be added here that since this paragraph was written I
have been very fortunate to secure numerous records of paths swam
by blindfolded swimmers, which strikingly resemble those of persons
walking blindfolded as described above. Most of the common swimming
strokes were employed in these observations and occasionally several
strokes were employed in a single experiment. In a few cases the
spiral path was made up of over twenty turns, and in one case of
over fifty turns. A fuller discussion of these results does not seem
pertinent here, and must be deferred to a later date.
[8] Since this was written I have been able to examine the movement
of live sperm cells in a number of representative animals, including
the jellyfish _Aurelia_; the molluscs _Ostrea_, _Solemya_, _Pandora_;
the arthropods _Limulus_ and _Anisolabia_, and the vertebrates frog,
turtle, snake, cat, dog and man, with the result that all these
spermatozoa revolve on their long axes and swim in spiral paths
resembling those of flagellates. Owing to their minute size their
movements are made out only with great difficulty, but so far as could
be determined all the sperms of any one species turn on their axes in
the same way, that is, either right-handed or left-handed. Recently
there has also come to my notice the very informing paper of W. D.
Hoyt, 1910, in the _Botanical Gazette_, in which it is stated that
fern sperms of various species swim in spiral paths.
End of Project Gutenberg's Ameboid movement, by Asa Arthur Schaeffer