LECTURE XX
REGENERATION
Budding and division—Every theory of regeneration in the meantime only provisional, a mere 'portmanteau theory'—Regeneration not a primary character—Volvox—Hydra—Vital affinities—Planarians—Heteromorphoses—Enemies of Hydroid-colonies—Regeneration in Plants—In Amphibians—In Earthworms—Different degrees of regenerative capacity according to the liability of the part to injury—Different results of longitudinal halving in Earthworms and in Planarians—Regeneration in Birds—The disappearance of the power of regeneration is very slow—Morgan's experiments on Hermit-crabs—Autotomy in Crustaceans and Insects—Regeneration of the lens in Triton.
We have endeavoured to explain the handing on of the complement of heritable qualities from one generation to another as due to a continuity of the germ-plasm, and we assumed that the germ-cells never arise except from cells in the 'germ-track'; that is, from cells which are equipped, from the fertilized egg-cell onwards, with a complete sample of slumbering germ-plasm, and are thereby enabled to become germ-cells, and, subsequently, new individuals, in which the aggregate of inherited primary constituents implied in the germ-plasm can again attain to development.
We have now to consider other cases of inheritance in relation to the same problem—the origin of their hereditary equipment.
We know, of course, that new individuals may arise apart from germ-cells, that, in many of the lower animals and in plants, they may arise by budding and fission.
For both these cases the germ-plasm theory will suffice, with a somewhat modified form of the same assumption which we made in regard to the formation of germ-cells. The origin of a new individual by budding seems often, indeed, to proceed from any set of somatic cells in the mother animal; but somatic cells, if they contain solely the determinants controlling themselves, cannot possibly give rise to a complete new individual, since this presupposes the presence of all the determinants of the species. But as these determinants cannot be formed de novo, the budding cells must contain in addition to the usual controlling somatic determinants, idioplasm in a latent, inactive state, which only becomes active under certain internal or external influences, and then gives rise to the formation of a bud. The source of this accessory idioplasm must, however, be looked for only in the egg-cell.
In plants this bud-idioplasm must be complete germ-plasm, because the budding starts only from one kind of cell, the cambium-cells; but in animals in which—as it seems—it always proceeds from at least two different kinds of cells—those of the ectoderm and those of the endoderm—the matter is more complex. In this case these two kinds of cells will contain as bud-idioplasm two different groups of determinants, which mutually complete each other and form perfect germ-plasm, and only the co-operation of these two sets will give rise to the formation of a bud. I will not, however, go further into detail in regard to these relations, for the theory can do nothing more here than formulate what has been observed; it is hardly in a position to help us to a better understanding of the facts.
The case is not much clearer in regard to the processes which lead to the replacing of lost parts. The manifold phenomena of regeneration can also be brought into harmony with the theory, if we attribute to those cells from which the replacing or entire reconstruction of the lost part arises an 'accessory-idioplasm,' which, at least, contains the determinants indispensable to the building up of the part. It is possible that the assumed accessory idioplasm frequently contains a much larger complex of determinants, and that it depends on the liberating stimuli which, and how many of these, will become active.
If we take a survey of regenerative phenomena in the animal kingdom, it strikes us at once that the capacity is very different in different species, extraordinarily great in some and very slight in others. In general it is greater in lower animals than in higher, but, nevertheless, the degree of differentiation cannot be the only factor that determines the capacity for regeneration. That unicellular organisms can completely replace lost parts, that even a piece of an infusorian can reconstruct the whole animal if only the piece contain a part of the nucleus, we have already seen when discussing the significance of the nuclear substance. In this case the nucleus must contain the complete germ-plasm, that is, the collective determinants of the species, and these induce the reconstruction of the lost part, though they do so in a way that is still entirely obscure to us. In the meantime, our interpretation will not carry us further, either here or in regard to any other order of vital phenomena. To go further would be little short of propounding a causal theory of life itself; it would mean having a complete and real 'explanation' of what 'life' is. As yet no one has been able to claim this position. We can see the different stages through which every organism passes, and that they arise one out of the other; we can even penetrate down to the succession of those delicate and marvellously complex processes which effect nuclear and cell-division; but we are still far from being able to deduce, except quite empirically, from the present state of a cell what the succeeding one will be, that is, from being able to understand the succession of events as a necessary nexus which could be predicted. How a biophor comes to develop from itself the phenomena of life is quite unknown to us; we know neither the interaction of the ultimate material particles nor the forces which bring it about; we cannot tell what moves the hordes of different kinds of biophors to range themselves together in a particular order, what molecular displacements and variations arise from this, or what influence the external world has, and so forth. We see only the visible outcome of an endless number of invisible movements—growth, division, multiplication, reconstruction, and differentiation.
As long as we are so far from an understanding of life no theory of regeneration can be anything more than a 'portmanteau theory,' as Delage once expressed himself in relation to the whole theory of inheritance, a theory which is like a portmanteau in that one can only take out of it what has previously been put in. If we wish to explain the renewal of the aboral band of cilia in a Stentor, we first pack our trunk, in this case the nucleus of the Infusorian, with the determinants of the ciliated region, and then think of these as being liberated by the stimulus of wounding, and being brought to and arranged in the proper place by unknown forces to reconstruct the ciliary region in some unknown way. No one could be more clearly aware than I am that this is not an exhaustive causal explanation of the process itself. Nevertheless, it is not quite without value, inasmuch as it allows us at least to bring the facts together in rational order—in this case the dependence of the faculty of regeneration on the presence of nuclear substance—under a formula which we can use provisionally, that is, with which we can raise new questions. As soon as we ascend higher in the series of organisms the theory gains a greater value, for, while we leave altogether out of account any answer to the ultimate question, and thus renounce for the present the attempt to find out how the determinants set to work to call to life the parts which they control, we are brought face to face with other, in a sense, preliminary questions which we can solve, and the solution of which seems to me at least not entirely without value.
The first of these questions runs thus: Is the power of regeneration a fundamental, primary character of every living being in the sense that it is present everywhere in equal strength, independently of external conditions, and thus is an inevitable outcome of the primary characters of the living substance? Or is it, though primaeval in its beginnings, a phenomenon of adaptation, which depends on a special mechanism, and does not occur everywhere in equal extent and potency?
We have already become acquainted with some facts which must incline us to the latter view. The globular Alga-colonies of Volvox ([Fig. 63]) consist of two kinds of cells, of which only one kind, the reproductive cells, possess the power of reproducing the whole, the others, the flagellate, or, as we called them, somatic cells, being only able to produce their like, but never the whole.
New investigations which have been carried out by Dr. Otto Hübner in my Institute have placed these facts beyond doubt. We may conclude that, in this case, a disintegration of the germ-plasm has taken place during ontogeny, by means of differential cell-division, so that only the reproductive cells receive the complete germ-plasm, while the somatic cells receive only the determinants necessary to their own specific differentiation, the somatic determinants.
In this case regeneration and reproduction coincide; there is no regeneration except the origin of a new individual from a reproductive cell.
Fig. 35 B (repeated). Hydra viridis,
the Green Freshwater Polyp.
Section through the body-wall,
somewhere in the direction of ov
in Fig. 35 A. Eiz, the ovum lying in
the ectoderm (ect), and including
zoochlorellæ (schl) which have immigrated
from the endoderm (ent)
through the supporting lamella
(st). After Hamann.
Let us now ascend to the lowest of the Metazoa, for instance, the freshwater polyp, Hydra ([Fig. 35 A]), and we find a high degree of regenerative capacity in the restricted sense, for, in addition to the power of producing germ-cells, that is, cells which, when two combine in amphimixis, give rise again to a new animal, almost any part of the polyp can regrow a whole animal. Not only has Hydra been cut in from two to twenty different pieces, but it has even been chopped up into innumerable fragments, and yet each of these, under favourable circumstances, was able to grow again into a complete animal. Nevertheless, we are not justified in concluding that every cell possesses the power of reproducing the whole. If, with the help of a bristle, we turn one of these polyps outside in like the finger of a glove, and then prevent it turning right again by sticking the bristle transversely through it, it does not live, but soon dies, obviously because the cells of the two layers of the body, ectoderm and endoderm, cannot mutually replace each other, and cannot mutually produce each other. The inner layer, now turned outwards, cannot resist the influence of the water, and the outer layer, now turned inwards, cannot effect digestion; in short, one cannot be transformed into the other, and we must therefore conclude that both are specialized, that they no longer contain the complete germ-plasm, but only the specific determinants of ectoderm and endoderm respectively.
The animal's high regenerative capacity must therefore depend on the fact that certain cells of the ectoderm are equipped with the complete determinant-complex of the ectoderm, in the form of an inactive accessory idioplasm, which is excited to regenerative activity by the stimulus of wounding, and that, in the same way, the cells of the endoderm are equipped with the whole determinant-complex of the endoderm. It need not be decided whether all or only many of the cells, perhaps the younger ones, are thus adapted for regeneration; in any case a great many of them must be distributed throughout the whole body, with perhaps the exception of the tentacles, which are by themselves unable to reproduce the whole animal. When the animal is mutilated, the cells of both layers, equipped with their respective determinant-aggregates, co-operate in reproducing the whole from a part.
It is true that even with these assumptions we only reach the threshold of a real explanation. For, given that all the determinants of the species must be present in a fragment, we are not in a position to show how these set about reconstructing the animal in its integrity, and the most that we can say is, that it must depend on the specific kind of stimulus to which each of the cells is exposed through its direct and more remote environment, which determinants are to be first liberated, and therefore which parts are to be reconstructed.
That there are at work regulative forces, such as we were already compelled to assume in regard to the division and regeneration of unicellular organisms, as to the nature of which we cannot yet make any definite statement, but which we may call 'polarities,' or, as I prefer to say, 'affinities,' is shown by countless experiments which have been made, particularly with the freshwater polyp. Thus Rand cut off the anterior end of the polyp with its circle of tentacles, and the excised disk of living substance lengthened in a transverse direction, so that half the tentacles came to lie to the right, the other half to the left, while the body developed between these two groups, so that they became further and further separated from each other, till finally the original transverse axis of the animal became the longitudinal axis. One group of tentacles survived and surrounded the new mouth, while the other at the opposite aboral pole, the new foot, died off. This total change of structure in the polyp, as to the arrangement of its main parts, points to unknown forces, which cannot depend on the determinants as such, but on the vital characters of the living parts, and on the interactions of these with one another.
Fig. 96. A Planarian cut transversely into nine pieces. The regeneration of seven of these into entire animals is shown. After Morgan.
The same holds true of all the lower Metazoa that have highly developed regenerative capacity, not only of polyps, but of worms such as the Planarians. Through the experiments of Loeb, Morgan, Voigt, Bickford, and others, we know that these animals respond to almost every mutilation by complete reconstruction, that they may, for instance, as is indicated in Fig. 96, be cut transversely into nine or ten pieces with the result that each of these pieces grows again to a whole animal, unless external influences are unfavourable and prevent it.
Something similar happens if the head be cut off a Tubularia-polyp, it forms a new head with proboscis and tentacles. It does so, at least, if the stalk of the polyp be left in the normal position; but if it be stuck into the sand in the reverse position a head arises at the end which is uppermost, where the roots arose previously, and the previous head-end now sends out roots. By suspending a beheaded stalk horizontally in the water a head can be caused to develop at each end of the stalk, so that we must assume that every part of the polyp is, under some circumstances, capable of developing a head, and that it must be 'circumstances'—in this case gravity, contact with earth or with water, and the mutual influence of the parts of the animal upon each other—which decide what is to be produced. Loeb, who was the first to observe this form of regeneration, called it heteromorphosis, to express the fact that particular parts of the animal might be produced at quite different places from those originally intended for them.
It would certainly be erroneous to range these cases of heteromorphosis against the determinant theory, but they certainly do not afford any special evidence of its validity as an interpretation, for all that we can say here again is that all, or at least many, cells of the animal must contain the full determinant-complex of the ectoderm, and others those of the endoderm, and that particular groups of determinants become active when they are affected by certain external or internal liberating stimuli. In regard to such animals the theory is hardly more convincing than the rival theory, that the faculty of regeneration is a general property of living substance, which does not attain to equally full expression everywhere, because it is met by ever-increasing difficulties involved in the increasing complexity of structure. The validity of the theory only begins to be seen when we deal with cases where it is demonstrable that every part cannot bring forth every other, where the power of regeneration is limited, and occurs only in definite parts in a definite degree, and can only start from particular parts. Here the assumption of a general and primary regenerative capacity fails. Any one who insists, as O. Hertwig does, that the idioplasm in all cells of the body is the same, can always plead that, in the cases in which regeneration does not occur, the fault lies, not in the regenerative capacity, but in the absence of the adequate liberating stimuli, and at first sight it does seem as if this position were unassailable. We shall find, however, that there are facts which make Hertwig's interpretation quite untenable.
My own view is that the regenerative capacity is not something primary, but rather an adaptation to the organism's susceptibility to injury, that is, a power which occurs in organisms in varying degrees, proportionate to the degree and frequency of their liability to injury. Regeneration prevents the injured animal from perishing, or from living on in a mutilated state, and in this lies an advantage for the maintenance of the species, which is the greater the more frequently injuries occur in the species, and the more they menace its life directly or indirectly. A certain degree of regenerative capacity is thus indispensable to all multicellular animals, even to the highest among them. We ourselves, for instance, could not escape the numerous dangers of infection by bacilli and other micro-organisms if our protective outer skin did not possess the faculty of regeneration, at least so far that it can close up a wound and fill up with cicatrice-tissue a place where a piece of skin has been excised. Obviously, then, the mechanism which evokes regeneration must have been preserved in some degree and in some parts at every stage of the phyletic development, and must have been strengthened or weakened according to the needs of the relevant organism, being concentrated in certain parts which were much exposed to injury and withdrawn from other rarely threatened parts. Thus the great diversity which we can now observe in the strength and localization of the regenerative capacity has been brought about. But all this can only be regarded as adaptation.
I should like to submit a few examples to show that the regenerative capacity is by no means uniformly distributed, and that, as far as we can see, it is greater or less in correspondence with the needs of the animal, both in regard to the whole and to particular parts.
It must first be pointed out that those lower Metazoa, like the Hydroid polyps in particular, which are endowed with such a high and general power of regeneration, do actually require this for their safety; they are not only soft, easily injured and torn, but they are most severely decimated by many enemies. In the beginning of May I found on the walls of the harbour at Marseilles whole forests of polyp-stocks of the genera Campanularia, Gonothyræa, and Obelia, all large and splendidly developed, with thousands of individual polyps and medusoids, but in a very short time the great majority of the polyps were eaten up by little spectre-shrimps (Caprellids) and other crustaceans, worms, and numerous other enemies, and towards the end of May it was no longer possible to find a fine well-grown colony. It must therefore be of decisive importance for these species if the stems and branches, which are spared because protected by horny tubes, possess the faculty of transforming their simple soft parts into polyp-heads, or of giving off buds which become polyps, or even of growing a new stock from the twigs which have been half-eaten and bitten loose from the stock and have fallen to the ground. If, finally, a torn-off polyp-stalk (of Tubularia) falls to the ground with the wrong side up, the end which is now the lower will send out roots, and the end now uppermost will give off a new head. This also appears to us adaptive, and does not surprise us, since we have been long accustomed to recognize that what is adapted to an end will realize this if it be possible at all. Think again of the innumerable adaptations in colour and form which we discussed in the earlier lectures. I hope later to be able to show in more detail how it comes to pass that necessity gives rise to adaptation. In regard to the case of the polyps, we can understand that, as far as a high degree of regeneration and budding was possible in these animals at all, it could not but be developed. Regeneration and budding complete each other in this case, for the former brings about in the individual 'person' what the latter does in the colony, namely, a Restitutio in integrum. It is readily intelligible that the former was not difficult to establish where the latter—the capacity of budding—was already in existence.
It seems at first sight very striking that the higher plants, which all depend upon budding, and which form plant-colonies (corms) in the same sense as the polyps form animal-colonies, only possess the faculty of true regeneration in a very low degree, although they are extremely liable to injury.
We see from this that the two capacities are not co-extensive, that germ-plasm may be contained in numerous cells of the body in a latent state, and yet that regeneration of each and every detailed defect may not be possible. This is the case in the higher plants in regard to most of their parts. A leaf in which a hole has been cut does not close the hole with new cell-material; a fern frond from which some of the pinnules have been cut off does not grow new ones, but remains mutilated. Even leaves which, if laid on damp earth, readily give off buds which grow to new plants, as the Begonias do, do not replace a piece cut out of the leaf; they are not at all adapted to regeneration.
From the standpoint of utility this is readily intelligible. It was, so to speak, not worth Nature's while to make such adaptations in the case of leaves or blossoms, partly because these are very transient structures, and partly because they are rapidly and easily replaceable by the development of others of the same kind. Moreover, the leaf in which we have cut a hole continues to function, but the polyp whose mouth and tentacles we have cut off could no longer take nourishment unless it were adapted for regeneration. But that this adaptation could have been made in the case of plants is proved by the root-tips which are formed anew when they are injured, and the closing of wounds on the stem by a 'callus.'
I shall return to plants when we are dealing with the mechanism of regeneration, but I must now direct more attention to animals, inquiring further into the question as to whether the faculty of regeneration is correlated with the degree of liability to injury to which the animal is exposed, and with the biological importance of the injured part, for this must be the case if regeneration be really regulated by adaptation.
Hardly any other vertebrate has attained such celebrity on account of its high regenerative capacity as the water-newt, species of the genus Triton. It can regrow not only its tail, but the legs and their parts if they are cut off. Spallanzani saw the legs grow six times, after he had cut them off six times. In the blind newt (Proteus) of the Krainer caves, a near relative of the common newt, the leg regenerated only after a year and a half, although the animal stands on a lower stage of organization than the newt, and thus should rather replace lost parts more easily. But Proteus lives sheltered from danger in dark, still caves, while Triton is exposed to numerous enemies which bite off pieces from its tail or legs; and the legs are its chief means of locomotion, without which it would have difficulty in procuring food. It is different with the elongated eel-like newt of the marshes of South Carolina, Siren lacertina. This animal moves by wriggling its very muscular trunk, after the manner of an eel, and in consequence of the disuse of its hind legs it has almost completely lost them. Even the fore-legs have become small and weak, and possess only two toes, and these do not regrow if they are bitten off, or only do so very slowly.
Earthworms are exposed to much persecution; not only birds, such as blackbirds and some woodpeckers, but, above all, the moles prey upon them, and Dahl has shown that moles often lay up stores of worms in winter which they have half crippled by a bite, while even Réaumur knew that moles frequently only half devoured earthworms. It was thus an obvious advantage to earthworms that a part of the animal should be able to regrow a whole, and accordingly we find a fairly well-developed regenerative capacity among them. But it varies greatly in the different species, and it would be interesting if we knew the conditions of life well enough to be able to decide whether the faculty of regeneration rises and falls in proportion to the dangers to which the species is exposed. Unfortunately we are far from this as yet; we only know that, in the common earthworms of the genera Lumbricus and Allolobophora, the faculty of regeneration is still very limited, for at most two worms, and sometimes only one, can develop from an animal cut into two pieces. Cutting into a greater number of pieces does not yield a larger number of worms, but usually only one, and often none at all.
This corresponds to the behaviour of their enemies, which may often bite off a piece or tear it away when the worm attempts to escape, but never cut it up into pieces. The regenerative capacity is more highly developed in the genus Allurus, more highly still in the worms of the genus Criodrilus which lives in the mud at the bottom of lakes, and most highly of all in the genus Lumbriculus which lives at the bottom of small ponds. Long ago Bonnet cut up a specimen of Lumbriculus into twenty-six pieces, of about two millimetres in length, and he observed most of these grow to complete worms again. His experiments have often been repeated in recent times, and have been extended and made more precise in many ways. Von Bülow was able to get whole animals from pieces consisting of from four to five somatic segments, and with eight or nine segments he almost invariably succeeded. A Lumbriculus which he had cut into fourteen pieces, one of which only measured 3.5 mm. in length, gave rise to thirteen complete worms with head and tail; only one piece perished.
These worms have little enemies with sharp jaws which may gnaw at them behind or before but cannot swallow them whole. Lyonet, famous for his analytic dissection of the wood-caterpillar (Cossus ligniperda), observed when he was feeding the larvæ of dragon-flies with these Lumbriculid worms that 'the anterior end of some whose posterior end had been gnawed away by the larvæ continued to live on the ground.' We can thus understand why a high power of regeneration is of use to these worms, and at the same time why it is advantageous to them to contract so that they break in pieces on very slight irritation, but to this we shall refer again.
The very diverse potency of the faculty of regeneration in animals belonging to the same small group, and nearly, if not quite, at the same level of organization, seems to show clearly that we have here to do with adaptation to different conditions of life, although we cannot demonstrate this in detail. It would certainly be erroneous to regard the conditions of life as uniform, since the worms in question not only live in different places—in the earth, in mud, or in water—and are thus exposed to different enemies, and since they may also be quite different in regard to size and speed, in means of defence, and possibly also of defiance, as is indeed in some measure demonstrable.
We meet with the same thing in a group of still smaller worms, Rösel's 'water-snakelets,' species of the genus Nais. These, too, behave in a variety of ways in the matter of regeneration, for while many species, such as Nais proboscidea and Nais serpentina will, if cut into two or three pieces, become two or three worms respectively, Bonnet expressly mentions an unnamed species of Nais which does not bear cutting up at all, and even dies if its head be cut off.
Thus neither the degree of organization nor the relationship alone determines the strength of the regenerative capacity. And as nearly related species may behave quite differently in this respect, so also do the different parts of one and the same animal; and here, too, the strength of the capacity seems to depend on the more frequent or rarer injury of the relevant part and on its importance in the maintenance of life. Let us take a few examples.
Parts which, in the natural life of the animal, are never injured, show in many cases no power of regeneration. This is so in regard to the internal parts of the newt, whose regenerative capacity is otherwise so high. I cut half or nearly the whole of a lung away from newts anæsthetized with ether; the wound closed, but no renewal of the organ took place. The same thing happened when a piece of the spermatic duct or of the oviduct was cut away. It is true that the kidney enlarges in higher animals when a piece has been cut out, by the proliferation of the remaining tissues, but that is a mere physiological substitution, evoked by the increased functional stimulus, due to the accumulation in the blood of the constituents of the urine. Such substitution depends on the growth of parts already existing, and it occurs in man when one kidney is removed, for the other, as is well known, may then grow to double its normal size. This is mere hypertrophy of the part that is left, it is not regeneration in the morphological sense, and it is not comparable to the re-formation of a cut-off leg in the salamander, or of a head in the worm, where the growth is not a mere increase of the remaining stump, but a new formation. It would be regeneration if a new kidney developed from the remnants of the kidney-tissue, or, in the liver, if new lobes grew in place of those which were cut off. But neither of these things happens, and, as far as I am aware, nothing of the kind has ever been observed, nothing more than new formation of liver-cells through increase of existing ones; that, however, is not regeneration in the morphological sense.
I have referred to the slight power of regeneration possessed by the blind Proteus in regard to its legs or tail, and I connected this with the absence of enemies in its thinly peopled cave-habitat. But the same animal can regenerate its gills when these are bitten off, and this is probably associated with the habit that Proteus has, in common with other newts with external gills, of nibbling at its neighbour's gills. Thus, the power of regenerating the gills was retained even when the animals migrated to the quiet caves of Krain, and were thus secured from the attacks of other enemies.
In lizards, a leg which has been cut off does not grow again, but an amputated tail does, and this has quite a definite biological reason, since the active little animal will seldom be caught by the foot by any pursuer, but may easily be caught by the tail, which is far behind. Thus the tail is adapted not only for regeneration, but also for 'autotomy' that is, for breaking off easily when it is caught hold of.
We have already seen that some segmented worms have a very high regenerative capacity; yet every part cannot produce every other, and while, in Lumbriculus, any piece of from five to nine segments is able to grow a new head or tail, neither ten nor twenty nor all the segments together, if they are halved longitudinally, can reproduce the other half, and the cause of this inability does not lie in the fact that the animal is thereby hindered from taking food, for even the transversely cut pieces do not feed until they have grown a new head and tail. The reason must lie in the fact that the primary constituents for this kind of regeneration are wanting, and they are so because a longitudinal splitting of this cylindrical and relatively thin animal never occurs under natural conditions, and thus could not be provided against by Nature[1].
[1] Morgan maintains that this statement is incorrect, and that Lumbriculus is capable of lateral regeneration. But if we look into the matter more closely we find that all he says is, that small gaps made by cutting a piece out of one side are filled up again, while the cut pieces perish. If the whole animal be halved, according to Morgan, both halves die, or if a 'very long piece' be cut out of one side, not only this piece dies, but also 'the remaining piece.' There is thus, as I have said, an essential difference between the regenerative capacity of Lumbriculus and that of Planaria.
That regeneration of this kind could have been arranged for if it had been useful we learn from the Planarians among the flat worms, in which every piece cut out of the body, large or very small, from the middle, from the left side, or from the right side of the animal, grows into a complete Planarian. The animal can be halved longitudinally, as in Fig. 97, and each half will grow to a whole. This again is quite intelligible from the biological point of view, for these flat, soft, and easily torn animals are exposed to all sorts of injuries, and are, in point of fact, frequently mutilated by enemies which are unable to swallow them whole. Von Graaf not infrequently found examples of marine Planarians (Macrostomum) which lacked 'a part of the posterior end or the whole tail region as far as the food-canal,' and of species of Monotus he found 'very often' in May specimens with the posterior end split or broken off. Probably the persecutors of these flat-worms are some species of Crustacean, but, at any rate, so much is proved, that the Planarians have abundant opportunities of making use of their faculty of regeneration, and that the species gains an advantage from it in respect to its preservation.
Fig. 97. A, a Planarian,
which has been divided into
two by a longitudinal cut.
Each half can grow into
an entire animal. B, the
left half at the beginning
of the regenerative process.
C, the same completed. After
Morgan.
In contrast to this, worms which live within other animals, and are thus secure from mutilation, such as the familiar round-worms (Nematoda), have no power of regeneration at all, and do not survive either longitudinal or transverse division.
Until recently birds were regarded as possessing a very low degree of regenerative capacity, and, as a matter of fact, they cannot replace a leg or a wing wholly or in part; but, what is otherwise unheard of among higher vertebrates, they can renew the whole anterior portion of the skeleton of the face, the bill, and can indeed almost reconstruct it with new bones and horny parts. Von Kennel communicated a case of this kind in regard to a stork, and for a long time this remained an isolated case, but a few years ago Bordage showed that, in the cocks which are used in the Island of Bourbon for the favourite sport of cock-fighting, the bill is regularly renewed when it has been broken off or shattered. Quite recently Barfurth gave an account of a case of complete renewal of a broken bill in a parrot. Yet it should not astonish us that the bill in birds has such a high regenerative power, for of all parts in a bird it is the one that is most readily injured; with it the bird defends itself against its enemies and its rivals, masters its prey, and tears it to pieces, pecks holes in trees (woodpecker), or climbs (parrot), or digs and burrows in the ground, or builds its nest, and so on. That the faculty of regeneration could be developed to so high a degree in relation to this particular part of the body, while the rest of the very important but rarely injured parts do not possess it at all, again points to the conclusion that the faculty of regeneration has an adaptive character.
It does not affect matters to discover cases in which we cannot recognize this relation between the regenerative capacity of a part and its importance or its liability to injury. Such instances do not lessen the convincingness of the positive cases, since we do not know the exact conditions which may lead to the increase of regenerative capacity in a part, and, above all, since we do not know the rate at which such an increase may take place. If adaptation in general depends upon processes of selection, these processes must also be able to give rise to an increase in the power of regeneration. On the other hand, it by no means follows that the disappearance of a faculty of regeneration which was once present in a part, but which has become superfluous in the course of time, must take place immediately through natural selection. For it is the very essence of natural selection that it only furthers what is useful, and only removes what is injurious; over what is indifferent it has no power at all. Thus it follows that the faculty of regeneration, when it has once been present in a part, cannot be set aside by natural selection (personal selection), for it is in no way injurious to its possessor. If it gradually decreases and becomes extinct notwithstanding this, when it is of no further use, as seems to be to some extent the case in regard to the legs and tail of the blind Proteus, that must depend on other processes, on those which generally bring about the gradual disappearance of disused parts or capacities. We shall attempt to probe to the roots of these processes later on; for the present let it suffice us to know that, according to our experience, they go on with exceeding slowness, and that it has taken whole geological periods to eliminate the legs of the snake-ancestors so completely as has been done from the structure of most of our modern snakes, while the Proteus which migrated into the caves of Krain as far back as the Cretaceous period is indeed blind, but still retains its eyes under the skin, though in a degenerate condition.
Since the degeneration of disused parts and capacities goes on so slowly it need not surprise us that we meet many parts which still possess regenerative capacity, although they are protected from injury. Thus Morgan found that, in the hermit-crab, the limbs which are protected within the mollusc shell were quite as ready to regrow as those which are actually used for walking, and thus are exposed to possibility of attack, but this proves nothing against the conclusion we drew from the facts cited above, according to which the faculty of regeneration comes under the law of adaptation. For the disappearance of this faculty must take place very much more slowly than its growth. For instance, the development of the tail-fin of the whale has long been an accomplished fact, while the hind-legs of this colossal mammal, which were rendered useless by the development of the tail-fin, still lie concealed in a rudimentary state within the muscles of the trunk. Yet these limbs must have lost their significance for the animal exactly at the time that the tail-fin became more powerful. Thus the retrogression must have taken place more slowly than the progressive transformation.
It is clear, then, that the faculty of regeneration is not a primary character of living beings occurring uniformly in all species of equally high organization and in all parts of an animal in the same degree; it is a power which occurs in animals of equal complexity in as varying degrees as in their parts, and which is manifestly regulated by adaptation. Between parts with the faculty of regeneration and parts without it there must be an essential difference; there must be present in the former something that is wanting in the latter, and, according to our theory, this is the equipment with regeneration-determinants, that is, with the determinants of the parts which are to be reconstructed.
If this be really so it should be capable of proof, at least in so far that we should be able to establish that the power of completing or re-forming a damaged or lost part is a limited one, localized in certain parts and cell-layers. This can be actually proved, as may be seen from numerous cases in which the faculty of regeneration is associated with autotomy, that is, with the power of breaking off or dropping off a part of the body. Even in worms we find this power, as we mentioned before in speaking of the high regenerative capacity of Lumbriculus. This worm reproduces in summer by what is called 'schizogony,' that is, by breaking into two, three, or more pieces, and it does not seem to require a very strong stimulus, such as pressure of the end of the worm by the jaws of an insect larva, to start this rupture; it often follows from quite insignificant friction on the ground. Certainly the power of regeneration is so great in this animal that it is out of the question to talk of localizing the primary constituents of regeneration; almost every broken surface is capable of regeneration.
But this localization is well illustrated in Insects and Crustaceans, which possess the power of self-amputation in their appendages, especially in their legs. As far back as 1826 MacCullock observed this remarkable power in crabs, and described the mechanism on which it depends. When the leg is irritated, for instance when it is pinched at the tip and held fast, it breaks off at a particular place. This line of breakage lies in the middle of the short second joint (Fig. 98, A and B, s), just between the insertions of the muscles (me, mf, m) which extend from this line towards the extremity of the limb and in the opposite direction towards the body-wall. Between these muscle-attachments the external skeleton is thin and brittle, and forms a suture, s, which breaks through when the animal contracts the muscles of the leg convulsively, and thus presses the lower protuberance (a) against a projection (b) of the first upper joint. Crabs require to make a very considerable muscular exertion before they can throw off the limb, and therefore they can only do it when they are in full vigour.
Fig. 98. The leg of a Crab,
adapted for self-mutilation or autotomy.
A, the first three joints of the limb, I, II, III. s, the
suture, that is, a thin area on
the second joint which is predisposed
to breakage. mf, flexor
muscle, me, extensor muscle, both
inserted at the suture. B, the
entire leg with its six joints and
with the suture (s). Slightly enlarged.
After MacCullock.
We have here a quite definite structural adaptation of the parts to a 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.
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.