THE TRANSFORMATION OF ENDOPLASM INTO ECTOPLASM
Perhaps none of the factors influencing the streaming of the endoplasm
mentioned above exercises as profound and constant an influence as its
capacity to form ectoplasm. As has been intimated earlier (p. 3-9)
streaming as observed during locomotion is not supposed to be possible
at all unless accompanied by the formation of ectoplasm at the forward
ends of pseudopods, and its transformation into endoplasm at the
posterior end of the ameba. We may therefore next consider the rôle
ectoplasm plays in locomotion, and in some other fundamental activities
of the ameba.
In the first place it is necessary to define the word ectoplasm, for two
entirely different meanings are sometimes given to it. It is used often
to designate the clear non-granular layer of protoplasm which thinly
covers some of the commoner amebas, and is especially prominent in some
of the small species, where the larger part of the anterior end often
consists of protoplasm quite free from granules. The other use to which
the word is put is to designate the layer of protoplasm on or near the
outside of the ameba which is more or less rigid and motionless,
resembling the gel state of a colloid. It is the latter meaning that is
given the word as used in this discussion, while I shall follow Jennings
(’04) and other, earlier, writers in using the word hyaloplasm in
speaking of the outer clear layer. It may be necessary to add that
neither of these two words is strictly definable, for in some cases, at
least, hyaloplasm is not more rigid than the endoplasm, while in other
cases it is. Strictness of definition can, of course, come only as
investigation proceeds; and these words as well as the word endoplasm,
should not be taken as defining the properties of the substances to
which they refer, but only as labels.
The demonstration of the most conspicuous and important property of
ectoplasm in _Amoeba proteus_ is easily made. With the high power of
the microscope one focusses on the upper surface of an active pseudopod,
paying especial attention to the small crystals imbedded in the
protoplasm. These crystals, although they dance about slightly (Brownian
movement) and otherwise change position to a slight extent, nevertheless
appear to be held in place by a very viscous medium. Such movement as is
observed in these crystals appears more or less erratic; it is not
coordinated and it is only by chance in the direction of locomotion of
the ameba. While observing the practically stationary crystals of the
ectoplasm one can at the same time, though indistinctly, see the forward
sweep of the crystals and other granules in the endoplasm below. But
observation fails to detect a definite line of separation between the
stationary ectoplasm and the mobile endoplasm; the one grades off
insensibly into the other.
The formation of ectoplasm in _proteus_ is a much more complicated
process than in almost any other ameba, excepting the large species
_Amoeba carolinensis_[1] discovered by Wilson (’00). We shall have
occasion however to refer at length to the method of ectoplasm formation
in _proteus_ later on, so we may consider _proteus_ first from this
point of view, and then take up a few other species in which the process
is simpler.
It is a fact more or less familiar to observers of amebas that
_proteus_, as distinguished from the other amebas, has a number of large
irregular, roughly longitudinal folds or ridges on its pseudopods and on
its main body (Figure 3). Under normal conditions these are never
absent. They are not found at the free ends of advancing pseudopods, but
they take their origin at some little distance from the ends. It is this
characteristic of ridge formation that complicates the process of the
transformation of endoplasm into ectoplasm; for instead of having to
deal with ectoplasm formation at the anterior ends of pseudopods only,
we find this process taking place irregularly all over the surface of
the ameba.
These folds or ridges were first observed by Leidy (’79) and it is an
eloquent tribute to the keen observation of this
[Illustration: Figure 3. Formation of longitudinal ridges and grooves in
the ectoplasm of _Amoeba proteus_. _A_, _B,_ _C_, _D_, showing stages in
the development of a single pseudopod. _a_, _b_, _c_, _d_, _d_^{1},
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 a certain 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 the
_ectosarc_. 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 in _proteus_ have 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 in _proteus_ in 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 section being 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 retains its 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 (See Figure 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 of _proteus_ and _carolinensis_, where ridges
are formed, with _discoides_ (Figure 11, _B_), _dubia_ (Figure 11, _C_),
_laureata_ (Figure 4) and _annulata_, where none are formed, the only
conclusion presenting itself is that the visible physical properties of
the protoplasm of _proteus_ and _carolinensis_ give no hint as to the
cause of the presence of ridges in these species. The protoplasm of
_discoides_ and _laureata_ is about as viscous as that of _proteus_, yet
in these there is never any ridge formation.
The ridges in _proteus_ recall, of course, the ridges always observed in
_verrucosa_, _sphaeronucleosus_ (Figure 13) and their congeners,
especially while the latter are in locomotion. A _sphaeronucleosus_ is
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 in _proteus_
and _carolinensis_. The chief difference lies in the fact that in
_sphaeronucleosus_ the ridges are extended 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. In _sphaeronucleosus_ and 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 in
_sphaeronucleosus_, _verrucosa_ and the rest of this group, is much
firmer than in most other amebas.