HISTORICAL SKETCH
For the purpose of presenting in brief compass the main published
observations and experiments on ameboid movement, we may pass from the
observations of v. Rosenhof, mentioned in the introduction, to certain
observations which Wallich (’63) recorded. He found that a new pseudopod
is usually formed as a small break in the ectoplasm somewhere on the
ameba through which the endoplasm then flows. As the endoplasm flows out
and the new pseudopod enlarges, the breach in the ectoplasm increases in
extent, through a transformation of the ectoplasm in the immediate
vicinity of the breach, into endoplasm. But he observed also that some
of the endoplasm which flows into the new pseudopod becomes transformed
into ectoplasm. Wallich thus demonstrated that ectoplasm and endoplasm
are mutually convertible.
The conversion of ectoplasm into endoplasm and vice versa, was regarded
by Wallich, however, as a process taking place only occasionally, such
as when new pseudopods are formed. It remained for Bütschli (’80, p.
115) to point out that in a moving ameba endoplasm is continually formed
from ectoplasm at the anterior ends of all pseudopods, while the reverse
process, viz., the conversion of ectoplasm into endoplasm, takes place
continually at the posterior end of the ameba. He describes the relation
of ectoplasm to endoplasm as a “circulation”; the endoplasm, arriving at
the anterior end, becomes changed into ectoplasm, which after remaining
relatively stationary for a while on the outer side of the animal, soon
finds itself at the posterior end of the ameba, where it is slowly
changed into endoplasm. The movement of the endoplasm forward to the
anterior end of the ameba completes the cycle.
In 1898 Rhumbler, from observations on several species of amebas, came
to the conclusion that in the _change_ from ectoplasm into endoplasm,
and vice versa, must be sought the cause of ameboid movement.
Jennings (’04), however, from extended study of the physiology of the
ameba, stressing especially movement and feeding, denied that the
transformation of endoplasm into ectoplasm, and vice versa, is necessary
or even of frequent occurrence during movement. Instead of these
transformations occurring regularly, as Bütschli and Rhumbler described
them, Jennings concluded that the ectoplasm is more or less permanent,
behaving like an elastic skin, which rolls over and over as the ameba
moves along. The ectoplasm thus remains ectoplasm, and the endoplasm
retains its identity, for considerable periods of time, instead of being
continually transformed, the one into the other, as the ameba moves
along.
Although observations with regard to movement in ameba have consisted
almost wholly of the mutual relations of ectoplasm and endoplasm, it is
important to note that the existence of a _third layer_ of protoplasm,
outside of the ectoplasm, was foreshadowed by an observation of Bütschli
(’92, p. 219) while examining a pelomyxa. To his great surprise he found
that there were currents of water, as evidenced by the movement of
suspended particles, at the sides and in close contact with the
ectoplasm of the pelomyxa, which flowed slowly forwards toward the
anterior end. No details were given and no explanation offered for the
cause of the currents excepting the suggestion that there might be a
thin skin over the animal, which moves slowly forward.
Two years later Blochmann (’94) demonstrated by means of the very fine
cilia-like projections which frequently cover the outside of pelomyxas,
that the surface of the pelomyxa actually moves forward during active
locomotion. He did not state definitely whether or not he considered
this surface as a part of the ectoplasm.
This observation of Blochmann was not developed, however, until Jennings
(’04), by means of particles attached to the outer surface of amebas,
studied the forward movement of this layer. The results of Jennings’
work led him to conclude that the outer surface of amebas, which move
forward as demonstrated by attached particles of soot and other
substances, is continuous with the ectoplasm, and is really the
ectoplasm. The rate of movement of this layer was stated to be about the
same as that of the ameba as a whole. He denied the validity of
Bütschli’s suggestion that there might be a thin third layer on the
outside of amebas or pelomyxas.
But the existence of a third layer of protoplasm as distinct from the
ectoplasm, was again maintained by Schaeffer (’17) who found that in
some amebas the outer surface moves forward faster than the ameba
advances through the water. The third layer was found to be generated
over the surface of the ameba, especially in the posterior region of the
ameba, and destroyed at the anterior end.
But the purely observational aspect of the problem of ameboid movement
has not interested biologists generally as much as the ultimate cause of
the phenomenon.
The first attempt that was made to explain ameboid movement in
conformity with the demands of modern experimental science, that is, on
the basis of physical factors, was made by Berthold (’86). By means of
simple experiments with inert fluids (oils, alcohol, water, ether) which
were modeled after an experiment described by the physicist Paalzow
(’58), Berthold concluded that locomotion in ameboid organisms is due to
the physical attraction of the anterior end to the substratum. The ameba
was supposed to behave like a drop of fluid which moved towards the
point where the tension of the ameba’s surface was decreased by contact
with the substratum. The ameba did not _push out_ pseudopods according
to Berthold, but they were _pulled out_ because of a difference in
surface tension between them and the substratum. But pseudopods which
were extended into the water and out of contact with a solid substratum,
were said to be extended by a contractile effort of the posterior region
of the ameba.
Bütschli (’92, p. 187) pointed out that it was highly improbable that
pseudopods in contact with a solid substratum were projected in a
fundamentally different way from that in which free pseudopods were
extended, as explained by Berthold. Bütschli assumed that all ameboid
movement was due to the same fundamental cause. He postulated surface
tension as the active agent, as Berthold had done for the extension of
pseudopods in contact with a solid substrate; but Bütschli assumed that
the decrease in surface tension at the anterior end of the ameba was
brought about by the bursting of protoplasmic droplets of a more fluid
consistency on the surface of the ameba, the consistency of which was
less fluid, thus bringing about a decrease of surface tension and
consequent forward streaming of the endoplasm. The necessary migration
of the more fluid droplets to the surface was determined by internal
conditions. The direction in which an ameba moves was assumed to depend
therefore not upon the physical character of the substrate, as suggested
by Berthold, but upon such internal changes as control the movement of
the more liquid part of the internal protoplasm to the outer surface.
Rhumbler (’98) wrote extensively on the subject of ameboid movement,
especially from the point of view of the feeding habits of amebas. He
concluded that the flow of protoplasm, while engulfing a food object,
was a direct result of the lowering of the surface tension of the
protoplasm by contact with the food object (p. 207), thus causing its
envelopment. Numerous other writers of the time, including Quincke
(’88), Verworn (’89, ’92), Blochmann (’94), Bernstein (’00) and Jensen
(’02), agreed in a general way with Rhumbler’s position that surface
tension changes are the cause of locomotion in ameba.
In 1904 the general subject of ameban behavior was extensively studied
by Jennings, and from his observations he concluded that surface tension
cannot account for many of the reactions observed. Other factors, he
held, must be at work, such as contractility, which, acting in the
posterior region, causes the endoplasm to flow forward. But Jennings
found it impossible to explain on the same basis the extension of free
pseudopods, and the creeping of a pseudopod, or of the whole ameba, over
a solid substratum.
From further observations Rhumbler (’05, ’10) came to modify his earlier
views as stated above. The rapid advances in the study of the chemistry
of colloids doubtless suggested to Rhumbler that the change from
endoplasm to ectoplasm resembled the change from a sol to a gel state,
and that in this process of gelation lay the source of energy manifested
in ameboid movement. In thus calling attention to, and emphasizing the
colloidal nature of, the conversion of endoplasm into ectoplasm and vice
versa, the problem of ameboid movement came to be discussed from an
entirely new angle. Certain phases of Rhumbler’s theory are developed
and elaborated by Hyman (’17) who agrees in general with Rhumbler’s
conclusions.
In a series of papers on feeding and other reactions of ameba, Schaeffer
(’12, ’16, ’17) concluded that Rhumbler’s general statement, wherein he
says that changes in behavior are directly deducible from the action of
stimuli in effecting liquefaction or gelation of the ectoplasm, does not
hold in many cases of feeding, and that the mechanism controlling
locomotion and feeding is not external, as maintained by Rhumbler, but
internal.