danger which often recurs--that of falling entirely into the power of
an enemy which has seized the leg. By a sudden violent throwing-off of
the leg the crab escapes from this danger. Quite similar adaptations
are found among certain insects, such as the walking-stick insects
or Phasmids, in which the mechanism is much the same, and lies at an
almost exactly corresponding place, namely, at the line where the
second and third joints of the leg, the 'trochanter' and the 'femur'
meet. In this case the advantage of the arrangement is not merely that
the animals are thus enabled to escape from enemies; it is useful in
another connexion, for a knowledge of which we have to thank Bordage.
This naturalist observed that the Phasmids not infrequently perished at
one of their numerous moultings, by remaining partially fixed in the
discarded husk. Of 100 Phasmids nine died in this way, twenty-two got
free with the loss of one or more legs, and only sixty-nine survived
the moult without any loss at all.
That the moulting or ecdysis of insects is often hazardous may be
observed in our own country, and it is familiar to every one who has
reared caterpillars. These, too, often fail to get clear of their
'cast' cuticle, and they perish unless artificial aid is given to them.
I have never observed any autotomy in them, but in the Phasmids it
seems to be a much-used 'device' and is therefore of great importance
in the persistence of the species.
Limbs which are thus thrown off by autotomy regenerate again from
the place at which they broke off, that is from the 'suture.' It
had been noticed even by the earlier observers (e.g. Goodsir) that
there was a jelly-like mass of cells within the joint, and that the
development of the new limb started from this. It might be supposed
that the regeneration-primordium is present in the rest of the leg
also, but that is not the case, for the animal responds to the tearing
off of one joint or of a smaller number than to the suture, not by
regenerating the torn part directly, but by amputating the whole of
the leg up to the suture, and then from this the regeneration of the
whole leg takes place. In the Phasmids the case is similar, but with
the difference that regeneration is possible from three places, from
the tarsal joints, from the lower third of the tibia, and finally,
from the suture between the femur and the trochanter. There is thus
a regeneration-primordium (_Anlage_) at the beginning of the tarsal
joints, another in the tibia, and a third in the 'suture' and the first
must be equipped, as we should express it, with the determinants of
the five tarsal joints, the second with those for the lower end of the
tibia as well, and the third with all the determinants of the whole
leg, from the 'suture' downwards.
In any case, regeneration is here associated with definite localized
pieces of tissue, and is not a general character of all the cells of
the leg, and, as it obviously runs parallel at the same time with
another adaptation--that of autotomy--there can be no doubt that it too
is dominated by the principle of selection, and that it can not only
be increased, but that it can be concentrated at particular places and
removed from others. But this is only possible if it be bound up with
material particles which may be present in or absent from a tissue, and
which are therefore a supplement to the ordinary essential constituents
of the living cells, although they do not themselves belong to the
essential organization.
I might cite many more examples of localization of regenerative
capacity, but will confine myself to one other, which seems to me
particularly instructive, because it was first interpreted as an
indication of the existence of an adaptive principle in the organism,
a principle which always creates what is useful. I refer to the
regeneration of the lens in the newt's larva.
G. Wolff, an obstinate opponent of the theory of selection, attempted
to solve the same problem as I had before me in my experiments on the
regeneration of the internal organs of newts, that is, he tried to
answer the question whether organs which are never exposed to injury or
to complete removal in the conditions of natural life, and which could
not therefore have been influenced in this direction by the processes
of selection, are nevertheless capable of regeneration. He extirpated
the lens from the eye of Triton larvæ, and saw that in a short time it
was formed anew, and from this he concluded that there was here 'a new
adaptiveness appearing for the first time,' and that therefore adaptive
forces must be dominant within the organism. The current theory of
the 'mechanical' origin of vital adjustments seemed to some to be
shaken by this, and the proclamation of the old 'vital force' seemed
imminent. And in truth, if the body were really able to replace, after
artificial injury, parts which are never liable to injury in natural
conditions, and to do so in a most beautiful and appropriate manner,
then there would be nothing for it but at least to regard the faculty
of regeneration as a primary power of living creatures, and to think of
the organism as like a crystal, which invariably completes itself if it
be damaged in any part. But we have to ask whether this is really the
case.
What makes the regeneration of the lens seem particularly surprising
is the fact that in the fully formed animal it must arise in a manner
different from that in which it develops in the embryo, that is, it
must be formed from different cell-material. In the embryo it arises
by the proliferation and invagination of the epidermic layer of
cells to meet the so-called 'primary' optic vesicle growing out from
the brain--a mode of development which cannot of course be repeated
under the altered conditions in the fully developed animal. The
reconstruction of the organ must therefore take place in a different
way, and if the organism were really able, the very first time the lens
was removed, to react in a manner so perfectly adapted to the end,
and so to inspire certain cells, which had till then had a different
function, that they could put together a lens of flawless beauty and
transparency, we should have reason to suspect that nearly all our
previous conceptions were erroneous, and to fall back upon a belief in
a _spiritus rector_ in the organism.
But the excision of the lens in these experiments was not by any means
an unprecedented occurrence! It is true enough that newts in their
pools are not liable to an operation for cataract, but it does not
follow that the lens is never liable to injury, and could not therefore
be adapted for regeneration. It can be bitten out along with the rest
of the eye by water-beetles or other enemies, and as far back as the
time of Bonnet and Blumenbach (1781) it was known that the eye of the
newt would renew itself if it were cut out, given that a small portion
of the bulb was left. But if this were removed the possibility of
regeneration was at an end. Thus, before the first artificial excision
of the lens, a regeneration-mechanism must have existed, by means of
which the eye with its lens was reconstructed, and this depends on the
characters of the cells of the eye itself--it is localized in the eye,
and without the presence of a piece of eye-tissue no regeneration can
take place. Is it then so especially remarkable that the lens should
be renewed when it is artificially removed without the rest of the
eye? The mechanism for its renewal is there, and is roused to activity
whether the lens alone or other parts of the eye also be removed.
We do not need, therefore, to assume the existence of a purposeful
or adaptive force; it is more to the point to inquire where the
regeneration-mechanism which suggests this inference is to be found.
A definite answer to this is given in a detailed experimental work
recently published by Fischel. It corroborates what Wolff had already
found, that the substance of the new lens develops from cells which
cover the posterior surface of the iris, that is, from cells of the
retinal layer of the eye. First, the margin of the pupil begins to
react to the stimulus of the injury (extraction of the lens); its cells
enlarge, become clear, while previously they were filled with dark
pigment, and finally they proliferate. They thus form a cell-vesicle
similar to the ectoderm-vesicle from which the lens arises in the
embryo, and into this the already mentioned retina-cells from the
posterior wall of the iris grow, elongate, and arrange themselves to
form the so-called 'lens-fibres,' on whose form, arrangement, and
transparency the function of the lens depends. This is marvellous
enough, but not more marvellous than that a whole foot should grow
on the cut stump of a newt's leg, or that a whole eye should arise
from a residual fragment. Here, again, we do not know the processes
which cause the arrangement of the cells and their often manifold
locally-conditioned differentiations, in short, we do not know the
_essential nature of regeneration_. But, in the meantime, we can
endeavour to find out which cell-groups regeneration is bound up with
in particular cases, so as to know where the vital particles, the
'determinants,' which condition regeneration, are placed by nature.
[Illustration: FIG. 99. Regeneration of the lens in the Newt's eye.
_A_, section through the iris (_J_); from its margin and posterior
(retinal) surface the primordium of a new lens (_L_) has developed
after the artificial removal of the old one. _B_, section through the
eye after duplicated regeneration of the lens (_L_) from two areas of
the iris. _Gl_, vitreous humour. _J_, iris. _C_, cornea. _R_, retina.
After Fischel.]
In this case there can be no doubt on that point: they are the cells
on the posterior wall and the margin of the iris. And it is certainly
not the _absence_ of the lens which gives rise to its renewal, as would
necessarily be the case if it were due to the dominance of an adaptive
force. If the lens, instead of being excised, be simply pressed back
into the vitreous humour occupying the cavity of the eye, a new lens is
developed all the same from the irritated margin of the pupil. And if
by chance this margin has been irritated in two places while extraction
of the lens was being performed, then two small lenses will develop
(Fig. 99, _B_). Indeed, several may begin to develop at the posterior
wall of the iris, although they do not attain to full development;
mechanical irritation of any part of this cell-layer is responded to by
the formation of lenses. This surely disposes of the 'mystical nimbus'
which would dazzle us with a new force of life, always creating what
is appropriate. We have before us an adaptation to the liability of
newts' eyes to injury, which, like all adaptations, is only relatively
perfect, since under the usual conditions of eye injury it gives
rise to a usable lens, but under unusual conditions to unsuitable
structures. It is exactly the same as in the case of animal instincts,
which are all 'calculated' for the _ordinary_ conditions of life, but,
under unusual conditions, may operate in a manner quite unsuited to the
necessary end. The ant-lion has the instinct to bore backwards into the
sand, and he makes the same backward-pressing movements when placed
on a glass plate into which he cannot force the tip of the abdomen.
The same is true of the mole-cricket, which makes its usual digging
movements with the forelegs even on a plate of glass. The wall-bee
roofs over her cell when she has laid an egg in it, but she does so
even if the egg be taken out beforehand, or if a hole be made in the
bottom of the cell, so that the honey which is to serve the larva for
food when it emerges from the egg runs out (Fabre). Her instinct is
calculated for filling the cell _once_ with honey, and _once_ laying
an egg in it, because such disturbances as we may cause artificially
do not occur or occur very rarely in natural conditions. There are
countless facts of this kind, for every instinct and every adaptation
can, in certain circumstances, go astray and become inappropriate. This
should be considered by those who still persist in opposing the theory
of selection, for herein lies one of the most convincing proofs of its
correctness. Adaptations can only arise in reference to the majority of
occurrences, and variations which are only useful in an individual case
must, according to the principle, disappear again. Adaptation always
means the establishment of what is appropriate in an average number of
cases.
Therefore the inappropriate reaction of the margin of the iris to an
artificial double stimulus affords additional reason for regarding
regeneration as an adaptive phenomenon. If it were the outcome of
an adaptive force it could never be inappropriate; and if it were
the operation of a general and primary power of the organism it
would be exhibited by the nearly-related frog as well as by the
newt. But, in the frog, extraction of the lens gives rise only to a
sac-like proliferation of the cells of the iris margin, which form no
transparent lens, but an opaque cluster of cells, which destroys vision
altogether. It appears, therefore, that the frog no longer requires the
power its ancestors possessed of regenerating a lost lens.
LECTURE XXI
REGENERATION (_continued_)
Phyletic origin of the regenerative capacity--The liberating stimuli
of regeneration--Production of extra heads and tails in Planarians
(Voigt)--Regeneration in the Starfish--Atavistic regeneration in
Insects and Crustaceans--Progressive regeneration--Regeneration has
its roots in the differentiation of organisms--The nuclear substance
of unicellular organisms is the first organ for regeneration--The
ultimate roots of regeneration.
In the previous lecture we have considered many different forms of
regeneration, and have recognized them as adaptive phenomena; we have
now to inquire how such regeneration-adaptations have arisen, and this
is a very difficult question even in general, while in particular
cases it is often quite unanswerable at present. In regard to the
case last discussed, the regeneration of the lens in the eye of
Triton, our hypotheses would require to reach back to the time of the
primitive vertebrates with an unpaired eye, for the lens of the paired
vertebrate eye, from Mammals down to the lowest Fishes, does not arise
in embryonic development from the retinal cells, but always from the
corneal epithelium, as the elaborate researches of Rabl have recently
shown. It is true that the unpaired parietal eye of some reptiles forms
its lens from the cells of the retinal layer, but it would be difficult
to demonstrate the possibility of a genetic connexion between it and
paired eyes, and in the meantime we must refrain from elaborating a
hypothesis as to the origin of the marvellous faculty the retinal cells
possess of transforming themselves into lens-fibres.
But it is easier to form some sort of picture of the origin and
adaptation of the faculty of regeneration in general.
We saw that the power of regenerating a part can be localized, and
that it does not belong to all the cells of the body, but only
to some of them, and we have to ask how and by what steps it has
been imparted to these. The faculty depends on the possession of a
regeneration-primordium (_Anlage_), and this again, in our mode of
expression, consists of a definite complex of determinants, and as
determinants are the products of an evolution, and thus are vital units
which have arisen historically, they can nowhere suddenly originate
anew in a species, but must be derived directly or indirectly from the
sole basis which, in each species, forms the starting-point of the
individual--that is to say, in the Metazoa, from the germ-plasm of the
ovum. From it the determinant-complex of every regeneration-rudiment
mast in the ultimate instance be derived.
We may think of the matter thus: all the determinants of the germ-plasm
vary, grow slowly or quickly, and in certain circumstances may be
doubled. In this way there arise what we may call 'supernumerary'
determinants, which are not required in the primary building up of
the body from the ovum, and which may remain in an inactive state in
the nuclei of certain cells, ready to become active under certain
circumstances and to produce anew the part which they control. Such
regeneration-idioplasm will at first come to lie in the younger
cells of the determinate organ, but it is conceivable that under the
influence of selection it may be gradually shifted to other cells of
a later developmental origin, or, conversely, to others in a less
external position, so that, for instance, the regeneration-rudiment for
the finger of a newt may be contained not merely in the cells of the
hand, but in those of the fore-arm or even of the upper arm.
But all such segregation of determinant-groups cannot have taken place,
as we might perhaps be inclined to think, at the periphery in the organ
itself during its development; it must take place in the germ-plasm of
the ovum, for otherwise it could not be transmissible, and could not be
directed and modified by the processes of selection, as is actually the
case, as I shall show in more detail later on.
I have already pointed out the importance of the rôle played by
liberating stimuli in regeneration, and not only of extra-organismal
stimuli, such as gravity, but above all of intra-organismal stimuli
that is, the influences exerted in a mysterious manner by other parts
of the animal on the parts which are in process of regeneration. It
is a great merit of the modern tendency in evolution theory that it
has demonstrated the importance of such internal influences. Although
we are still far from being able to define the manner in which these
influences operate, we may say so much, that it depends essentially on
the nature and extent of the loss which parts are reproduced by the
regenerating cells, and, also, on the position and direction of the
injured surface from which the regeneration starts. The influences,
still quite beyond our comprehension, which are exerted on the
regenerating part by the uninjured parts constitute the liberating
stimuli, which evoke the activity of one or other of the determinants
contained in the regeneration-idioplasm.
[Illustration: FIG. 100. Regeneration of Planarians. _A_, an animal
divided into three parts by two oblique cuts. _B_, the fragments(_a_,
_b_, _c_) in process of regeneration. _C_, an animal with various
oblique incisions in the margin of the body, which have induced the new
formation of heads (_k_), of tails (_s_), and pharynx (_ph_). _A_ and
_B_ after Morgan; _C_ after Walter Voigt.]
Walter Voigt has shown, by a series of most interesting experiments,
that it is possible not only to cause the development of a new head
in Planarians by cutting them, in which case a tail may grow from the
anterior portion and a head from the posterior portion, but it is
also possible in an intact animal, that is, one with both head and
tail, to cause the production of a second head, or a second tail, or
both at once, at any part of the body margin at will, according to
the direction of the cut. If the margin of the body be cut obliquely
forwards (Fig. 100, _A_) a supernumerary tail arises (_C_, _s_), if
it be cut obliquely backwards a supernumerary head arises (_C_, _k_),
and in this way several heads and several tails may be produced in
the same animal. It is obvious, then, that the interaction, in the
first place, of the cells of the cut surface, but probably also of the
deeper-lying cells, decides which determinants are to come into action,
those of the head or those of the tail, but both must be present at
every part of the cut. How far below the cut surface the cells take
part in this determination we cannot make out, but that it cannot be
due to the co-operation of all parts is clear in this case at least,
since the animal still possesses its original head and tail. The extra
heads and tails thus produced prove, at any rate, that there can be no
question here of the expression of an adaptive principle, a _spiritus
rector_, or a vital force, which always creates what is good, but that
it is rather a purely mechanical process, which takes its course quite
independently of what is useful or disadvantageous, and that it must
take this course according to the given regeneration-mechanism and the
stimulus supplied in the special case. It cannot be supposed that these
supernumerary heads and tails are purposeful, but who would expect an
adaptive reaction from the animal in a case like this, since cuts of
the kind which we make artificially, and _must keep open artificially_
if the deformities are to develop, hardly occur in nature, and, if they
did occur, would very quickly close up again? Adaptations can only
develop in response to conditions which occur and recur in a majority
of cases, and when they have a useful, that is, species-preserving
result. The adaptiveness of the organism is blind, it does not see the
individual case, it only takes into account the cases in the mass,
and acts as it must after the mechanism has once been evolved. The
case is the same as that of 'aberrant' or mistaken instincts, whose
origin by means of selection is the more clearly proved, since we must
recognize such an instinct as a pure mechanism and not as the outcome
of purposeful forces.
In the regeneration of Planarians we must think of the
regeneration-idioplasm as containing the full complex of the
collective determinants of the three germinal layers, and possibly
we must add to this cells with the complete germ-plasm for giving
rise to the reproductive cells. But when the amputated tail of the
newt is regenerated, or its leg, or the arm of a starfish, or the
bill of a bird, we have no ground for assuming that the cells, from
which regeneration starts, contain the whole germ-plasm, since the
determinants of the replaceable parts suffice to explain the facts.
We must even dispute the possibility of the presence of the whole
germ-plasm in this case, because the faculty of regeneration of the
relevant cells is really no longer a general one, but is limited to
the reproduction of a particular part. This is seen in the fact that,
in the starfish, whose high regenerative capacity is well known, the
central disk of the body may indeed give rise to new arms[2]; but
an excised arm, to which no part of the disk adheres, is in most
starfishes unable to give rise to the body. Thus the arm does not
contain in its cells the determinants of the disk, but the latter
contains those of the arm. We are not surprised that the amputated
tail of the salamander does not reproduce the whole animal, but this
can only be because the impelling forces to the regeneration of the
whole animal are wanting, that is, that the cut surface only contains
the determinants of the tail and not the complete germ-plasm. It might
be objected here that the tail-piece is too small to give rise to the
whole body, but in _Planaria_ it is only very diminutive heads and
tails which grow from the artificial incisions, and the same is true
of starfishes when only a single arm and a small piece of the disk
have been left. Notwithstanding the small amount of living substance
at their disposal, and although they are at first unable to take
nourishment, they send out very small new arms (Fig. 101), close up the
wounded surface, and, after reconstruction of the mouth and stomach,
begin to feed anew. The new arms may then grow to the normal size.
[2] I see now that there are contradictory statements in regard to this
case. Possibly these depend on the different behaviour of different
species, and this on the varying frequency of mutilation. Starfishes
which live on the shore between the rocks, for instance on the movable
stones of a breakwater, are very frequently mutilated; in some places
it is rare to find a specimen without traces of former wounds. H. D.
King counted among 1,914 specimens of _Asterias vulgaris_ 206 in the
act of regenerating a part, that is, 10.76 per cent. In the case of
the starfishes from deep water this cause of injury does not of course
exist.
[Illustration: FIG. 101. A starfish arm, growing four new arms; the
so-called 'comet-form.' After Haeckel.]