THEORIES OF REGENERATION
It is significant to find that the theory of pre-formation of the embryo
in the egg, that was so very widely held during the seventeenth and
eighteenth centuries, and during the first part of the nineteenth
century, was at once applied to the explanation of the regeneration of
animals when this process became known. Bonnet in 1745 attempted to
explain the newly discovered facts in regard to the regeneration of
animals by means of the pre-formation theory. Just as the egg was
supposed to enclose a pre-formed germ, so he imagined there lay
concealed latent germs in the adult animal. At first Bonnet thought that
these germs must be whole germs, like those contained in the germ cells
of the reproductive organs, and that only as much of any one developed
as was needed to replace the missing part. Later, however, he admitted
that the germs might be incomplete germs, which are so located in each
region that they represent the parts of the body beyond that region. The
_purpose_ of these germs is to replace any accidental injuries to the
animal. He pointed out that some animals are more subject to injuries
than others, and these animals are he thought especially well supplied
with germs. Since in some animals the same part may be replaced several
times, Bonnet assumed that on each occasion a new germ is awakened. As
many sets of germs are present in these animals as the number of times
the animal is liable to be injured in the course of its natural life.
Bonnet found that in lumbriculus a new head and a new tail may appear at
almost any level, if the worm is cut in two, and, therefore, he
supposed, head germs and tail germs are present throughout the worm. But
why, if this is so, should a head germ always develop at the anterior
end, and a tail germ at the posterior end of a piece cut from the body?
Bonnet’s keen mind saw that it was necessary to make a further
assumption. He supposed that the fluids of the body that pass forward
carry nourishing substances for the head. When the worm is cut in two
these substances are stopped at the anterior cut-surface, and there
accumulating act on the latent head germ, and nourishing it, cause it to
develop. Correspondingly the nourishing substances for the tail flow
backward, and accumulating at the posterior cut-surface awaken a tail
germ to activity.
The part of the body in which these nourishing substances are supposed
to be produced is not specifically stated, but in one passage Bonnet
says that the fluids that flow toward the head are there used up in that
organ, and we may infer that he held a similar view for the posterior
region. He offers no explanation of the cause of the flow of these
substances in a given direction, and in this respect his hypothesis
lacks support where it is most needed. In fact, it is no more improbable
that a head germ should always develop at the anterior end and a tail
germ at the posterior end, than that head-forming substances should flow
in one direction and tail-forming in another. It is not that it is worth
while to object to Bonnet’s hypothesis on the ground that it does not
explain everything, but it is worth while to point out that it gives
only the appearance of an explanation, and that it begs the whole
question by the assumption of particular nourishing fluids flowing in
definite directions. So far as the blood is concerned, we know that the
different parts of the body take from it those substances or fluids that
they make use of, not that special fluids flow to particular regions. It
is probable that Bonnet thought of the blood rather than of any other
subtler fluids passing through the tissues; and, if so, there is nothing
that we know in regard to the behavior of the blood that lends support
to Bonnet’s idea.
Bonnet takes care to state that the pre-formed germs may not appear to
us like miniature copies of the part into which they develop, but they
are so constructed that, as they absorb nourishment and become larger,
they assume a characteristic form.
Weismann, who has also accepted the pre-formation hypothesis to account
for the development of the egg, has applied the same conception of
pre-formation to the process of regeneration. He believes that partial,
latent germs are present in different parts of the body, and that these
germs are present especially in animals that are liable to injury and in
those parts of the body that are likely to meet with accidents. In these
essential respects, Weismann’s idea is the same as Bonnet’s; but in
regard to the location of the germs, and their manner of awakening, and
as to how the forms, liable to injury, have acquired their power to
regenerate, Weismann adopts more modern standards. He believes that the
germs are located in the nucleus. Those that bring about the development
of the egg are supposed to be different from those that bring about
regeneration, because the method of regeneration is generally different
from the method of development of the egg.
Regeneration, on Weismann’s view, is brought about by latent cells
containing pre-formed germs in the chromosomes of the nucleus. These
germs are called the determinants. Since at each level in an animal, or
in a part of an animal, regeneration may occur and replace the missing
part, it is assumed that the germs are correspondingly different at each
level, and represent all the parts that lie distal to that region.
Weismann does not suppose that there is a single germ at each level that
represents all the distal parts, but that in each layer, or organ, or
part there are many cells that contain germs corresponding to the distal
regions. The qualities of the latent cells are sorted out by means of
the qualitative divisions of the chromatic material of the nucleus.
Moreover, since the new part can itself regenerate, the further
assumption is made that during regeneration new subsidiary or latent
cells are laid down at each level. This is supposed to be brought about
by a quantitative division of each germ after it has reached its
definitive position in the new part.
Weismann’s general attitude toward the problem of regeneration is summed
up in the following statements: “It may, I believe, be deduced with
certainty from those phenomena of regeneration with which we are
acquainted, that _the capacity for regeneration is not a primary quality
of the organism, but that it is a phenomenon of adaptation_.” Again,
“Hence there is no such thing as a general power of regeneration; in
each kind of animal this power is graduated according to the need of
regeneration in the part under consideration.” “We are, therefore, led
to infer that the general capacity of all parts for regeneration may
have been acquired by selection in the lower and simpler forms, and that
it has slowly decreased in the course of phylogeny in correspondence
with the increase in complexity of organization, but that it may, on the
other hand, be increased by special selective processes in each stage of
its degeneration in the case of certain parts which are physiologically
important and at the same time frequently exposed to loss.”
The evidence brought forward in the preceding pages leads, I think, to
precisely the opposite conclusions, and, in certain cases at least, it
has been shown that there can be no relation between the power of
regeneration and the extent of exposure of a part to injury or to loss.
It is unnecessary to enter here further into this question, since it has
been discussed already in Chapter V.
Weismann’s statement that the power of regeneration has decreased “in
correspondence with the increase in the complexity of the part” cannot
by any means be entirely accepted. If the complexity of a part is of
such a kind that the part cannot sustain itself independently until
regeneration has taken place, or if the exposed surface of the wound is
such that it cannot be closed over, or if the new part cannot be
properly nourished, or if the tissues have changed in such a way that
their cells can no longer multiply, then the statement is, to a certain
extent, true. On the other hand, when we find that one of the most
complicated organs of the body, the eye, can regenerate in the
salamander, if only a piece of the optic cup is left attached to the
nerve, we may well doubt if there is any such direct and general
connection between regeneration and complexity as Weismann maintains.
Weismann’s so-called “mechanism” of qualitative nuclear division is the
basis of his conception of pre-formation. We are, I think, at present in
a position to reject not only this conception, since it finds no support
either in observation or experiment, but also his view that regeneration
is brought about by latent cells; for it has been shown in a large
number of cases that the new cells come directly from the old,
differentiated ones. In a previous chapter it has been pointed out that
Weismann’s idea that regeneration has been acquired by a process of
natural selection, and is under the influence of this supposed agent, is
in direct contradiction to a number of known facts. Under these
circumstances we are warranted, I think, in concluding that the entire
Weismannian point of view is wrong.
The process of regeneration has been often compared to the process by
which a broken crystal completes itself. Herbert Spencer, in particular,
has elaborated this idea. In his book on the _Principles of Biology_, he
says: “What must we say of the ability an organism has to recomplete
itself when one of its parts is cut off? Is it of the same order as the
ability of an injured crystal to recomplete itself? In either case new
matter is so deposited as to restore the original outline. And if, in
the case of a crystal, we say that the whole aggregate exerts over its
parts a force which constrains the newly integrated molecules to take a
certain definite form, we seem obliged, in the case of the organism, to
assume an analogous force.” Spencer has called attention to a
superficial resemblance between the renewal of a part of a crystal and
the regeneration of an animal, and without further inquiry into the
profound differences between the processes, assumes that “analogous
forces” are at work. Now that we know something more of both processes,
we find so much that is totally different, that there remains no basis
for Spencer’s conclusion, namely, that analogous forces must be present.
Furthermore, Spencer’s statement that the whole crystal aggregate exerts
over its parts a force of some kind is diametrically opposed to our idea
as to the method of “growth” of a crystal in a saturated solution. The
new material is added always at the surface of the crystal, and the
growth of each point is self-determining. There is no central force that
controls the deposition of new material in the different regions.
Rauber’s work on the so-called regeneration of the crystal has given us
a clearer conception of how the process is brought about. He has shown
that when a piece is broken from a crystal, and the crystal suspended in
a saturated solution of the same substance, it becomes larger by the
deposition of new material _over all its surfaces_. The addition of new
material may be more rapid over the cut-surface than elsewhere, but it
must not be supposed that the more rapid “growth” takes place in order
to complete the form of the crystal, for the growth over the cut-surface
follows precisely the same laws that regulate the “growth” over all the
other surfaces, that is taking place at the same time. In this respect
we find an essential difference between the regeneration of a crystal
and that of an animal, since in the latter the growth takes place only
over the cut-surface; and, in forms that regenerate by proliferation, at
the expense of the old material, so that the old material is
correspondingly diminished as the new part grows larger. Regeneration
may even take place in an animal deprived of all food, and also in one
that is starving to death and diminishing in size. In those forms that
regenerate by a change in shape of the entire piece into that
characteristic of the typical form, the process bears not even the
remotest resemblance to the process in the crystal. It is so obvious
from every point of view that the comparison is entirely a superficial
one, that it seems useless to point out further differences between the
two processes.
Pflüger (’83) has given, in brief outline, an hypothesis to account for
the process of regeneration. He states that since there is always
replaced exactly what is lost, the new part cannot arise from a
pre-existing whole germ. If, for instance, the leg of a salamander is
cut off at any level, as much comes back as is removed. The assumption
of a leg germ is insufficient to account for the fact that only as much
comes back as is lost, and not always a whole leg. Pflüger, therefore,
offers another hypothesis. He assumes that food material is taken up at
the wounded surface and organized into the substance of the new part.
The new material is laid down at the surface of the old material, and is
then organized into the kind of tissue that lay just beyond that region
in the whole limb. Upon this first layer a new layer is deposited that
is organized into the next part of the limb, and so on, until the whole
missing part is replaced. Pflüger does not give any idea of how the new
material is deposited at the cut-surface, but from what we know of the
histology of the process we must suppose, if we should adopt Pflüger’s
interpretation, that new cells are produced by the old ones, and that
these new cells form the successive layers out of which the new limb is
produced. Pflüger speaks of an arranging molecular force, which we can
only suppose, in the light of what has just been said, to act from cell
to cell through the continuous protoplasm. Pflüger also pointed out that
in certain cases the organization can take place only in a certain
direction, that is, in some forms regeneration can take place from one
side of a cut-surface, but not from the other. He interprets this as
due to a polarization of the protoplasm, one surface having
peculiarities that are absent in the other.
There are certain objections to Pflüger’s hypothesis that suggest
themselves. In the first place the new part does not, in many cases,
replace all that has been removed, and hence it is difficult to see how
the building up in the way Pflüger supposes, could take place. In these
cases the new material forms only the distal end of the part removed,
and the relation of the old to the new part is of secondary importance.
Again, in cases of heteromorphosis, as when a tail develops on an
anterior cut-surface of a piece of an earthworm, the result must be due
to quite different factors from those suggested by Pflüger. The results
are, in fact, the reverse of what the hypothesis demands. Furthermore,
when the entire piece is transformed into a whole new organism, there is
very little in the process to suggest a change like that postulated by
Pflüger. On the other hand there cannot be much doubt that the old part
may have some influence, and in certain cases a very important influence
on the new part, but whether this is a purely molecular influence is
open to doubt. In whatever way this influence may act, it is only one of
a number of factors that take a share in the result. The amount of new
material, that is formed before the organization of the new part begins,
seems to be also a factor; and the one that determines how much of the
missing part can be replaced, and this in turn seems to be connected
with the lowest organization size that can be produced. The distal end
of the new part forms always the distal end of the organ that is to be
produced. If enough new material has developed (before the organization
of the new part takes place) to produce all of the missing part, the
latter is formed, but if the material is insufficient to produce the
whole structure, then as much of the distal end as possible is formed.
In some cases, as in the planarians, the missing intermediate regions
may subsequently develop behind the distal part that is first produced.
Sachs has advocated a view which has many points of similarity to that
of Bonnet, although, in reality, it is not a theory of pre-formation at
all, but one of pure epigenesis. His idea rests on the view that the
form of a plant, or of an animal, is the expression of the kind of
material of which it is composed. Any change in its material leads to a
corresponding change in the form of the new parts. Sachs holds that the
idea of many morphologists, that there is for each organism a specific
form that tends to express itself, and which controls the development of
the organism, is a metaphysical idea that has no ground in science. For
instance, Sachs thinks that the flower buds of a plant develop, not
because of some innate, mystical force that causes the plant to
complete its typical form, but because some substance is made in the
leaves which, being carried into the growing region, becomes there a
part of the material of that region, and from this new material a flower
is formed. Simple and clear as this hypothesis appears to be at first
sight, it will be found on more careful examination that it fails to
account for some of the most characteristic phenomena of development and
of regeneration. It may be granted at the outset that the presence of
certain substances may undoubtedly influence the kind of growth of a new
part; but, on the other hand, one of the most characteristic things of
the organism is that it asserts its specific nature within quite a wide
range of change, and, on the whole, resists the influence of other kinds
of substances than those connected with its ordinary life. While Sachs
looked no farther than the material substratum, and supposed that any
change in this altered the form, there is, at present, sufficient
evidence to show that it is the _structure_ of the material that
determines the most important changes that take place in it. This means,
if we attempt to divest the statement of its somewhat metaphysical
appearance, that the material of the organism is not simply a mixture of
different kinds of materials, but a special kind of substance that has a
definite structure of its own. This structure may, of course, be
changed, but only by the addition of materials that the structure can
take up as a part of itself. If the material does not become a part of
the structure or organization, it is without effect on the form.[130] My
meaning can, perhaps, best be illustrated by the method of regeneration
of the tail of the fish from an oblique cut-surface. The growth of the
new part is not determined by the kind or by the amount of the new
material that is brought to the growing part, for, if it were, the new
part would grow at an equal rate at every point; but the growth of the
new part is regulated by the form of the tail of each particular kind of
fish. The structure of the new part controls the growth of the material
of the new part, and not the reverse. The only interpretation that can
be given to this result is, I believe, that the new material assumes a
definite structure, or what we may call an organization, and the
subsequent changes are controlled by the kind of structure that is
present; and since this structure has, as a whole, a definite form, we
can state that the form controls the material, although the substitution
of the word “form” for that of “the structure of the new material” may
give the statement an unfortunate, metaphysical appearance.
In order to explain the regeneration of a piece of a plant, Sachs
supposes that two substances are produced by the plant,--one a stem-(or
leaf-) forming substance and the other a root-forming substance. If
either of these substances combines with the protoplasm of any part, a
stem or a root is produced from that part. When a piece of the stem is
cut from a plant, these two substances accumulate, one at the distal end
and the other at the proximal end of the piece, and their presence in
these regions determines that new shoots develop at or near the apex,
and new roots at the base. Sachs tried to show that the direction of the
flow of these two substances is determined by the action of
gravity,--the lighter substance flowing to the higher parts, and the
heavier to the lower parts. We find here reproduced Bonnet’s idea of
specific substances flowing in definite directions; but Sachs goes
farther, and gives an explanation of the cause of the different
directions taken by the two kinds of substances, viz. that it is due to
the action of gravity. Vöchting has shown, as we have seen, after a
thorough examination of the method of development of pieces of plants,
that Sachs’s hypothesis fails to account for the results; and he shows
also that an internal factor, which he calls the polarization, has the
most important influence on the regeneration.
It is not difficult to show that there are many other cases to which the
stuff hypothesis does not apply. If, as Bonnet attempted to show, the
regeneration is due to different stuffs, there is no explanation to
account for the flow in animals of head-forming stuffs forward and
tail-forming stuffs backward. In animals that regenerate laterally as
well as anteriorly and posteriorly, we should be obliged to assume
side-forming stuffs as well as head-forming and tail-forming stuffs; and
since the kind of structures that are produced at the side are different
at each level, we should be obliged to assume that there are many kinds
of lateral stuffs. If regeneration can take place in a dorsal and in a
ventral direction, as, for example, when the dorsal and the anal fins of
teleostean fishes regenerate, there must also be stuffs to account for
their development. When regeneration takes place from an oblique
surface, it must be supposed that two or more of these kinds of stuff
are brought into action. The regeneration of just as much of the limb of
the salamander as is cut off also offers difficulties for Sachs’s view.
If we assume a leg-forming substance, it fails to account for the
difference in the result at each level. If we assume that different
substances come into play according to the amount of the leg that has
been cut off, the hypothesis becomes as complicated as the facts that it
pretends to explain.
A special case, to which the stuff hypothesis has been applied by Loeb
and by Driesch, is that of tubularia, although the latter writer has
used the hypothesis only to a limited extent as involving quantitative
rather than qualitative results. There is present in the hydranth and
stem of tubularia a red pigment in the form of granules in the
endodermal cells. There is more of the red pigment in the stem near the
hydranth than elsewhere. If a piece of the stem is cut off, it closes
its cut-ends, and a circulation of fluid begins in the central cavity.
In this fluid globules now appear that contain the red-pigment granules.
The globules appear to be free endodermal cells, or parts of such, that
have been set free in the central cavity. In the course of twenty-four
hours the new hydranth begins to appear near one end of the stem, and in
this region of the stem a much larger number of granules appear. A
little later all the red granules disappear from the circulation.
Driesch has supposed that the red granules of the circulation become a
part of the wall of the new hydranth. The disappearance of the red
granules at this time from the circulation would seem to give color to
this view. But, on the other hand, I have found evidence showing that
this interpretation is incorrect. In the first place, the granules that
disappear from the circulation can be found lying in a ball within the
digestive tract of the newly formed hydranth; hence their disappearance
can be accounted for, and we find that they are not, or at least in
large part are not, absorbed into the forming hydranth.[131] In the
second place, there is a great increase in the number of endodermal
cells in the region in which the hydranth is about to appear, and the
thickening that results takes place some time before the granules begin
to disappear from the circulation. The new granules appear in the new
endodermal cells, and are presumably formed by them. Again, the
hydranth, that develops later at the distal end, appears when there are
no granules in the circulating fluid, and yet the hydranth may contain
as much red pigment as does the proximal one. Lastly, the development of
very short pieces shows that at the time of the formation of the new
hydranth there is an enormous increase in the number of red granules in
the piece, for there are many more of them contained in the new hydranth
than were present in the entire piece at the time of its removal.
Loeb has not referred to the red granules in the circulating fluid, but
simply to the red pigment which is present in the walls of the piece.
This is supposed to move forward into the hydranth region, and call
forth the development of a new hydranth. A study of the number of the
granules in the stem gives no support to this idea, and the method of
formation of single and of double hydranths in short pieces shows that
the increase in the number of granules in the hydranth-forming region is
not due to migration, but to local formation.
That specific substances may have an influence on the growth of certain
parts cannot be denied, but it appears that in general they play a very
secondary rôle as compared with other factors that determine the form
of the organism or the development of a part. Vöchting’s beautiful
experiments (’86) on tuberous plants show that the presence of an
excessive amount of food substances in the plant, brought about by the
artificial removal of the natural storehouses for such material, may act
on certain parts, such as the axial buds, or on the stem, and cause them
to produce structures that they do not produce under ordinary
circumstances. The axial buds become swollen and produce tuber-like
bodies above ground, especially if the parts are enclosed so as to be in
the dark, since the light retards the growth of tubers of all sorts. But
it should not be overlooked that these buds and stems are structurally
the same things as the tuberiferous stolons that have been removed, and
hence the excess of material is stored up in them in the same way as it
is under normal circumstances in the underground stems or stolons. The
reaction is one normal to the plant, although it usually takes place in
a different part.
The preceding hypotheses that have been advanced to account for the
phenomena of regeneration, draw attention to some of the most
fundamental problems of regeneration and, even in those cases in which
the hypotheses have not given a satisfactory solution of the problems,
some of them have served the good purpose, both of directing attention
to important questions and of leading biologists to make experiments to
test the new points of view. We should not underrate their value, even
if they have sometimes failed to give a solution of problems, for they
have been useful if only in eliminating certain possibilities, and this
simplifies all future work. So long as an hypothesis is of a sort that
it is within the range of observational and experimental test, it may be
of service, even if it prove erroneous; for our advance through the
tangled thread of phenomena is not only assisted by advances in the
right direction, but all possibilities must be tested before we can be
certain that we have discovered the whole truth. The value of a
scientific hypothesis depends, it seems to me, first, on the possibility
of testing it by direct observation, or by experiment; second, on
whether it leads to advance; and, lastly, on its elimination of certain
possibilities.
The experiments described in Chapters II, III, IV, have shown that there
are many resemblances between the phenomena of growth and of
regeneration. It has been pointed out that when it could be shown that
certain external agents have a determining influence upon growth, these
same agents have a similar effect upon regeneration. This also holds
apparently for internal factors, although it is much more difficult to
demonstrate that this is true. The presence of an abundance of food
material in the tissues hastens regeneration in the same way that growth
is more rapid in a well-fed organism. Food may, however, be looked upon
rather as an external factor than as an internal one. An excellent
example of an internal factor is found in the interrelations of the
parts to each other. This is shown in the development of a piece of a
plant in which the apical buds develop faster than the proximal ones,
and it appears that, in some way, the development of the latter are held
in check by the development of the apical ones. Another case is found in
the development of the bilobed tail of certain fish in which particular
regions are held in check, while others grow at the maximum rate.
It is a curious fact that while we can cite several kinds of external
influences that affect the development and the regeneration of
organisms, the only internal factors that have been discovered are the
so-called polarity and this interrelation of the parts. Perhaps there
should also be added the specific nature of certain parts, limiting the
possibilities of new growth in these parts, and the presence of the
nucleus as necessary for the growth and regeneration of the organism.
If it be admitted that the same factors that affect the growth also
affect in the same way the regeneration, we have made a distinct
advance. It is, moreover, not difficult for us to understand how this is
possible. If we consider first those cases in which growth takes place
at one or more points at which the cells are undifferentiated, and
compare this condition with that in regenerating animals that produce
new tissue by proliferation, we can picture to ourselves that the same
factors would act on the undifferentiated tissue in the same way in both
cases. This does not explain what causes the organism to produce the new
cells that appear over an exposed surface, and we must search for other
factors to account for the out-wandering of cells, and for the local
multiplication of the cells at the cut-end. We find a parallel to those
cases in which the growth of an organism takes place throughout the
whole body, in those animals in which the regeneration also takes place
in the old part. This comparison should not, however, be pushed too far,
since, in some forms, as, for example, a salamander, the growth of the
animal takes place throughout the body, while regeneration takes place
by the proliferation of new material. The difference in the regenerative
process in a salamander and in a form like hydra is not due so much to
the inability of the old cells of the salamander to increase in number
as compared with those of hydra, but rather, it appears, to a certain
rigidity or stiffness of the body of the salamander that prevents the
rearrangement of the parts; and the recompletion of the form takes place
in the direction of least resistance, _i.e._ at the open or cut-end of
the body.
Regeneration by means of morphallaxis takes place only in those forms in
which the body is not made up of a series of separated parts. This kind
of regeneration occurs in those organisms in which the normal growth
consists only in the enlargement of a system of organs already present.
A piece of an animal of this sort usually contains the elements of each
kind of organ, and from these the new parts are produced, both by
proliferation at the cut-ends and by the enlargement of the parts that
are present in the piece. In forms with separate segments we find, in
some cases, resemblances between normal growth and regeneration, as
shown, for example, in the earthworm. There is present in the young worm
a region in front of the last segment, or, rather, a part of this
segment, from which new segments are formed. In the regeneration of the
posterior end a knob of new tissue is formed, and out of this a few
segments develop, the last one having a growing region similar to that
in the young worm. The subsequent stages in the regeneration involve the
formation of new segments from the last one, as in the young worm. There
is no such growing zone at the anterior end of the young worm, and none
is formed in the regeneration of an anterior end, so that only the
segments that are first laid down in the new part are present in the new
anterior end.
An interesting comparison may be made between the phenomenon of growth
and that of contraction and expansion of the protoplasm. The bending of
heliotropic organisms toward or away from the light, and the similar
bending of negatively stereotropic forms away from contact with a solid
body, are supposed to be phenomena of growth, and resemble in many ways
the phenomenon of contraction. In a plant that bends toward the light,
it is found that the most obvious change involves the amount of water on
the two sides of the stem, and this is most probably connected with a
fundamental structural change in the protoplasm, that is too subtile for
further analysis. In the regeneration of some forms it is found that
they respond in the same way to light. While it cannot be demonstrated
that these phenomena really depend on processes of contraction and of
expansion, the results are nevertheless suggestive from this point of
view. Furthermore, I think, one cannot study the regeneration of such
forms as planarians, hydras, stentors, etc., without being struck by the
apparent resemblance of the change in form that they undergo to a
process of expansion. The idea of the expansion of a viscid body carries
with it, of course, the idea of tension within the parts, and the return
to the former condition is brought about by a release from the tension
and a return to a more stable condition. If by the intercalation of new
material the extended condition is fixed, a new state of equilibrium
will be established.
It has been already pointed out that in a piece of a plant suspended in
a moist atmosphere, the apical buds are those that first develop, and
also grow faster than the others. The buds situated nearer the base may
not even begin to develop, although they are at first as favorably
situated, so far as external circumstances are concerned, as the
uppermost ones. The roots appear first over the basal end, and those
nearer the base grow faster than do those nearer the apex. There cannot
be much doubt that the suppression of the basal buds and of the more
apical roots is connected with the development of the apical buds and of
the basal roots. This can be shown by cutting a piece in two, when some
of the basal buds will grow into shoots and the apically situated
root-buds, that are now on the base of one piece, will begin to grow. It
seems to me this relation can be at least more fully grasped, if we look
upon it as connected with some condition of tension in the living part.
The tension can be thought of as existing throughout the softer, more
plastic parts. As long as the apical bud is present at the end of a stem
or branch, or even near the apex, it exerts on the parts lying proximal
to it a pull, or tension, that holds the development of these parts in
check; but if the apical bud is removed the tension is relaxed, and the
chance for another bud developing is given.
It may be asked, how can it be explained that only the more apically
situated buds of a piece develop, rather than the basal ones, since with
the removal of the piece from the plant the tension has been removed
also. The only answer that can be made, so far as I can see, would be
that from the apex of the plant to its base the tension is graded, being
least at the apex and increasing as we pass to the base. Those buds will
first develop that are in the region of least tension, and their
development will hold in check the other buds by increasing or
reëstablishing the tension on the lower parts of the piece. A new system
is then established, like that in the normal plant.
There are certain experiments with hydra that can, perhaps, be brought
under the same point of view. When two long posterior pieces are united
by their anterior cut-surfaces, each piece regenerates a circle of
tentacles near the region of union, and each may produce a new head; or
only one head, common to both pieces, develops at the side. Each piece
has retained its individuality, which may be interpreted to mean that
each piece has retained its original condition of tension. If, however,
after a union of this kind one piece is cut off, as soon as the two have
well united, near the place of union, so that it is relatively small as
compared with the other component, it may produce a head at its exposed
basal end, and neither heads nor tentacles may develop at the place of
union of the pieces.
It is probable, in this case, that the larger component has acted on the
smaller one, so that its polarity is changed and becomes like that of
the larger component. It is possible, I think, to interpret this result
in terms of our tension hypothesis. The condition of tension in the
larger piece has overcome that of the smaller piece, so that the latter
comes to have the same orientation that the larger piece has; and the
development of a head at the free end then takes place. The development
of this head holds in check the development of a head at the anterior
end of the larger piece in the region of union of the pieces. When two
pieces of hydra are united by unlike poles, _i.e._ so that they have the
same orientation, it is found that if the pieces are not too long, a
head develops at the free end and none in the region of grafting. The
result is similar to that in plants; the development of the head at the
free end suppressing any tendency that may exist to produce a new head
by the posterior piece at the place of union. If the pieces united in
this way are very long, a head develops at the apical end, and, in some
cases, also near the line of union. This may be due to the pieces being
so different at the place of union, that a head develops below this
region before the unification of the two pieces is brought about, or
because the formation of the head at the free end is relatively so far
removed from the place of union of the pieces, that it does not
influence the development of a head in this region.
These cases of grafting also illustrate another point of some interest.
They show that the development of a head at the anterior end of a piece
is not the result of the injury from the cutting or due to the action of
some external condition on the free end, for the regeneration may take
place when two anterior ends have been perfectly united to each other.
The result can only be explained as the outcome of some internal factor
such as polarity.
These examples have been chosen from hydra rather than from tubularia,
in which somewhat similar phenomena have been observed, because in hydra
the development of heteromorphic structures is of rare occurrence, while
in tubularia external influence often calls forth a heteromorphic
development. There cannot be much doubt, however, that in tubularia the
same kind of internal factors are also at work.
A more striking illustration of the possible influence of tension of the
parts is shown by an experiment with planarians. If the head of a
planarian is cut off and the posterior piece is split partially in two
along the middle line, as shown in Fig. 31, _A_, and then one of the
halves is cut off just anterior to the end of the longitudinal cut, the
result is as follows: A new head develops at the anterior end of the
long half (Fig. 31, _B_), but no head develops on the posterior
cut-surface, provided this part has reunited along the middle line with
the long half, and a line of new tissue connects the anterior
cut-surface of the long half and the more posterior cut-surface of the
shorter half. At least this happens if the piece is not split too far
posteriorly, _i.e._ through the region of the pharynx. If this is done,
a new head may develop from the posterior cut-surface. In another way
the development of the more posterior head can be brought about. If the
shorter side-piece is kept from fusing with the longer side-piece in the
middle line, it will invariably produce a new head (Fig. 31, _C_). The
lack of development of the posterior head, when the two cross-cut
surfaces are united by a connecting part of new material, can, it seems
to me, be best explained by the influence of the developing anterior
head, or of the new side on the posterior new tissue, and this influence
can, I think, be better appreciated if we suppose some sort of tension
to be the influence at work.
Another example may be cited that shows even more clearly that the
internal factor regulating the growth in the new part is probably some
sort of tension. I refer to the development of the tail of fundulus from
an oblique cut, or of the bilobed tail of stenopus from a cross cut. The
assumption of the typical form that leads to the holding in check of the
growth in certain regions, as compared with others, can be best
understood, I think, as due to some sort of tension established in the
different parts, that regulates the growth in those regions.
It is evident that whatever factor will serve to explain the preceding
cases must also be expected to apply to the development of the whole
embryo from parts of the egg or blastula, if the position that I have
taken is correct, namely, that these phenomena belong to the same
general group. Does the tension hypothesis make clearer the development
of a whole embryo from a part of an egg? This means, can we think of the
readjustment that takes place as due to the establishment of a
characteristic equilibrium that expresses itself in the tensions of the
different regions? There is, so far as I can see, no difficulty in
supposing that the organization is at bottom a system of this kind;
indeed, it seems to me that from this point of view we can get a better
appreciation of the organization and of the series of changes that take
place in it during development. The example that Driesch has chosen as a
typical one of vitalistic action, namely, the proportionate development
of a part of the archenteron of the half-embryo, seems to me to be
likewise a case to which we can apply the tension view.
In these, as well as in all other cases, we must think of the tensions
as existing, not only in one direction, but in the three dimensions of
space, and of all combinations of these. The material in which the
tensions exist must be thought of as labile, so that a change in one
region involves a rearrangement in many cases of the entire system. The
new rearrangement appears to take place on the foundations of the old
system.
It may appear that this idea of a system of tensions is too vague, that
it fails to point out how the reorganization takes place, and that it
gives not much more than the facts do themselves. There is a certain
amount of truth in these objections which I fully appreciate, but
something further can be said on these points. The view is vague in so
far as we cannot picture to ourselves in a mechanical way just how such
a system could bring about the suppression of growth in one region and
allow the maximum amount in another region. But this is asking too much,
since the hypothesis can only claim, at present, to furnish a means by
which we can at least imagine what sort of a process is involved, and
cannot give the details of the process itself. It can be shown
experimentally that if the phenomenon is one of tension certain results
should follow that are observed to take place, as when by keeping the
shorter half of the planarian from reuniting to the larger half, or by
breaking the union if it has been formed, a head develops also at the
posterior cross-cut. In the second place, although we cannot understand
how the rearrangement of the tensions in a piece takes place, yet from a
causal point of view we can see how a change in one region of a labile
system may produce, by means of a change of tension, a complete
rearrangement of the parts throughout. It can even be claimed for the
tension hypothesis that it at least becomes easier for us to see how
such a change could take place, because it represents the organization
as the expression of a system under tension, and hence, if the material
is sufficiently flexible, a readjustment will probably take place when
the system is changed in any region. It enables us to see how the
organization of the egg may be divided by every cell division, and yet
after the reunion of the cells the original equilibrium be established.
We may perhaps claim, therefore, that in these respects the hypothesis
does give us something more than do the facts; and, inasmuch as it
brings a large number of phenomena under a common point of view, the
idea may be worth further consideration.
In conclusion, I may add that the hypothesis is, I hope, also a
legitimate one, in the sense that being within reach of an experimental
proof or disproof, it may serve at least as a working hypothesis.
Perhaps more fundamental than the idea that a system of tensions exists
throughout the organization is the conception that the organization is
itself a system of interrelated parts, and not a homogeneous substance
or a mass composed of a large number of repeated parts, or rather,
despite the presence of smaller, repeated units, the organization is not
the result of their interaction, but of their regular arrangement as
parts of a whole structure. If, then, this interrelation of the
different parts of the structure can be looked upon as the result of a
system of tensions, we can at least form a better idea as to how a piece
of a whole can readjust itself into a new whole of smaller size. And it
is this possibility of rearrangement or regulation that is one of the
most characteristic properties of living things.