Regeneration

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 outwandering 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 mace 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 onJy 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 reestablishing 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 halfembryo, 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 systern. 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 inter-

relation 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. IN the preceding chapters certain matters had to be taken for granted, since it was not possible, or desirable, at the time to discuss more fully some of the terms that are in common use, or to analyze more completely many of the phenomena. It was also not necessary to give the general point of view under which the phenomena were considered in their physical, chemical, or even causal connection. Little harm has, I trust, been done by relegating such questions to the final chapter. An attempt will now be made to give more explicit statements in regard to the use and meaning of such terms as " organization," " polarity," " factors," " formative forces," " vitalistic " and "mechanical principles," "adaptation," etc.

It will be found that the hypotheses that have been advanced to account for the phenomena of development and of regeneration may be roughly classified under two heads : first, those in which the organization is " explained " as the result of the collective action of smaller units ; and second, those in which the organization is itself regarded as a single unit that controls the parts. Let us examine these points of view more in detail, in order to see what has been meant in each case by "the organization."

A favorite method of biological speculation in the last forty years has been to refer the properties of the organism to invisible units, and to explain the action of the organism as the resultant of their behavior. The hypothesis of atoms and of molecules, by means of which the chemist accounts for his reactions, has proved so exceedingly, fruitful as a working hypothesis that it has had, I think, a profound influence on the mind of many biologists, who have, consciously or unconsciously, attempted to apply a similar conception to the structure of living organisms. The discovery that all of the higher organisms are made up of smaller units, the cells, and that the lower organisms are single, isolated cells, comparable to those that make up the higher forms, has also drawn attention to the idea that the whole organism is the result of the action of its units. Furthermore, within the cells themselves units of a lower order have also been discovered, such, for instance, as the chromosomes, the chloro-

phyl bodies, etc., that repeat on a smaller scale some of the fundamental properties of the entire organism, as growth and division. It has been assumed that still farther down in the structure there are smaller units having the same properties, and the smallest of these are the ultimate units. The organism is looked upon as the result of the properties of these minute germs. The gemmules of Darwin furnish an example of an hypothesis of this sort ; also the intracellular pangens of De Vries, the plasomes of Wiesner, the biophors of Weismann, the idiosomes of Hertwig, and the micellae of Nageli are other examples of this way of interpreting the organization. These elements are endowed by their inventors with certain properties, and these are of such a sort that they give the appearance of an explanation to organic phenomena. It is useless to object to these hypotheses that they are purely ideal, or fictitious, and that those properties have been assigned to the germs that will bring about the desired explanation, and have not been shown to be the real properties of the germs themselves. But apart from the arbitrariness of the process, it cannot be claimed that a single one of these creations has been shown to be true, or has even been accepted by zoologists as probable. A more serious objection to this point of view is that the most fundamental characteristics of the organism, those that concern growth, development, regeneration, etc., seem to involve in many cases the organism as a whole. So many examples of this have been given in the preceding pages, that it is not necessary to go over the ground again. It has been shown that a change in one part takes place in relation to all other parts, and it is this interconnection of the parts that is one of the chief peculiarities of the organism.

In phenomena of this kind even the cells seem to play a secondary part, and if so, we can, I think, safely leave out of account the smaller units of which the protoplasm is supposed to be built up and we can neglect them, if for no other reason than this, that the argument that has called them into existence starts out with the cell as the highest unit. If the cell can be thrown out, most probably the units of which the cell itself is supposed to be made up can be safely disregarded also.

It may be objected that only through a knowledge of the minute structure of the organism can we hope to understand the behavior of the whole ; but my point of view is not that there may not be a fundamental structure, but that this is not formed by a repetition of elements, which give to the whole its fundamental properties. It can be shown, I think, with some probability that the forming organism is of such a kind that we can better understand its action when we consider it as a whole and not simply as the sum of a vast number of smaller elements. To draw again a rough parallel; just as the properties of sugar are peculiar to the molecule and cannot be ac-

counted for as the sum total of the properties of the atoms of carbon, hydrogen, and oxygen of which the molecule is made up, so the properties of the organism are connected with its whole organization and are not simply those of its individual cells, or lower units. The strongest evidence in favor of this view is found in the behavior of small pieces of an egg, or of a protozoon, or even of a many-celled organism. A lower limit of organization is very soon reached, below which the piece fails to produce the characteristic form, although all the necessary elements are present in the piece to produce the entire structure. The size of these pieces is enormously large as compared with the size of the cell, or of the imaginary elements of Nageli, Weismann, Wiesner, etc. These results indicate that the organization is a comparatively large structure.

A few writers have either ignored the presence of smaller units, or have dealt with the organism from a purely chemical and physical point of view. They attempt to account for the changes in the organism as the outcome of known physical and chemical principles. It must, of course, be granted that in a sense the properties of the organism are the result of the material basis of the organism ; but in another sense this idea gives a false conception of the phenomena of life, because, if we were simply to bring together those substances that we suppose to be present in the organism we have noreason to think that they would form an organism, or show the characteristic reactions of living things. Even from a chemical point of view we can see how this result could not be expected, for it is well known that the order in which a compound is built up, i.e. the way in which the atoms or molecules are introduced into the structure, is an important factor in the making of the compound. When we remember the immense period of time during which the organisms living at present have been forming, we can appreciate how futile it will be to attempt to explain the behavior of the organism from the little we know in regard to its chemical composition. Its chief properties are the result of its peculiar structure, or the way in which its elements are grouped. This structure has resulted from the vast number of influences to which the organism has been subjected, and while it may be granted that if we could artificially reproduce these conditions an organism having all the properties that we associate with living things would result, yet the problem appears to be so vastly complicated that few workers would have the courage to attempt to accomplish the feat of making artificially such a structure.

To prevent misunderstanding, it may be added that while from the point of view here taken, we cannot hope to explain the behavior of the organism as the resultant of the substances that we obtain from it by chemical analysis (because the organism is not simply a mixture of these substances), yet we have no reason to suppose that the organism is anything more than the expression of its physical and chemical structure. The vital phenomena are different from the non-vital phenomena only in so far as the structure of the organism is different from the structure of any other group of substances.

Nageli has stated that each part acts as though it knew what the other parts are doing. His idea of the idioplasm involves a conception of the organism as a whole and not simply the sum total of a number of parts. Hertwig, who maintained at one time that the development of the embryo is the resultant of the action of the cells on each other, admits in his work on Die Zelle nnd die Gewebe that while this is in part true, yet on the other hand the whole also exerts an influence on its parts. Driesch, who hypothetically suggested at one time that the nuclei act as centres of control of the cell by means of enzymes, has later adopted a widely different view. Whitman has made a strong argument to the effect that the cell theory is too narrow a standpoint from which to treat the organism, and on several occasions I have urged that the organism is not the sum total of the action and interaction of its cells, but has a structure of its own independent of that of the cells.

This discussion will suffice to show some of the opinions that have been held as to the nature of the organization of the organism. Let us next ask what properties we may ascribe to it. It has been found that certain polar, or rather dimensional, relations are characteristic of the organization. The term "polarity" expresses this in a limited way, but refers only to one line having two directions, while we now know that the dimensional properties relate to the three dimensions of space, and for this idea we might make use of the term heterotropy. Thus we find that a piece of a bilateral animal regenerates a new anterior end from the part that lay nearer the anterior end of the original animal, a new right side from the part that was nearest the original right side, and a new dorsal part from the region that lay near the original dorsal part, etc.

The polarity of a part can be changed in certain forms, as in tubularia, by exposing the posterior cut-end to the external factors that bring about the formation of a hydranth, or, as in hydra, by grafting in a reversed direction a smaller piece on a larger one. In Planaria lugubris and in the earthworm the polarity of the new tissue may be reversed, as compared with that of the part from which it develops, if the new part arises from certain regions of the body. A curious instance of the effect of the polarity is shown by the regeneration from an oblique surface in planarians. The new head arises from the more anterior part of the new material, rather than from the

middle of the anterior oblique surface, and the new tail arises from the more posterior part of the posterior oblique surface. As an analysis of this result has been already attempted in an earlier chapter, it will not be necessary to go further into this question here. The development of a new part at right angles to an oblique surface has also been described, and it has been pointed out that the result appears to be due to the symmetrical development of the new structure in the new part. This symmetry of the newly forming part must be also counted as one of the properties of the organization.

Finally, the mode of regeneration of a new, bifurcated tail in the teleost, stenopus, shows that the new part may very early become moulded into the characteristic form, and that the growth of the different parts is regulated by the structure assumed at an early stage. The new part does not grow out at an equal rate until it reaches the level of the notch of the old tail, and then continue to grow at two points to produce the bilobed form of the tail ; but the bilobed condition appears at the very beginning of the development.

These illustrations give us nearly all the data that we possess at present on which to build up a conception of the organization. That we must fail in large part fully to grasp its meaning from these meagre facts is self-evident. The main difficulty seems to lie in this, — that when we attempt to think out what the organization is we almost unavoidably think of it as a structure having the properties of a machine, and working in the way in which we are accustomed to think of machines as working. The most careful analysis of the " machine theory," as applied to the phenomena of development and of regeneration, has been made by Driesch. It has been pointed out that in his Analytische Theoric Driesch assumed that development is due to " given " properties in the egg ; that each stage is initiated by some substance contained in the egg acting on the stage that has just been completed. That is, each stage is the condition of the following. The "rhythm" of development is accounted for in this way. The changes are described as due to chemical processes (including also ferment actions). The nucleus is supposed to contain all the different kinds of ferments that act, when set free, as stimuli on the protoplasm ; but since the ferments are always set free at the propitious moment, Driesch was obliged to assume that the cytoplasm acts on the nucleus in such a way as to make it produce the proper ferment for the next stage. Thus the cytoplasm first influences the nucleus, the latter sets free a specific ferment that starts a new chemical change in the cytoplasm, and the changed cytoplasm may then react again on the nucleus, and a different ferment be set free, etc. Each change is therefore not only an effect of what has gone before, but the cause of the

next process.1 Driesch points out that it is necessary at this stage to make a further assumption, because the cytoplasm must not only be acted upon by the ferment, but it must itself be of such a sort that it responds to the action. This leads to a great complication of the phenomena ; but the assumption does not depart, in the last analysis, from the idea of the cell as a system in a mechanical sense. This assumption of a receiving and an answering station for the stimuli carries with it the further assumption of a many-sided "harmony." Without a harmony at each step in the development there could be no orderly ontogeny. The assumption of this harmony introduces a new element into the series of hypotheses. The appearance of a causal explanation was given in those parts of the argument preceding the introduction of the assumption of a harmony, but with the admission of this new element into the argument, the causal point of view is left. Driesch says in this connection : " If we cannot gain a singleness of view in the way that has been followed, we can reach this in another way. Indeed, the way of doing so has been already implied in that part of the theory dealing with the harmony of the phenomena. The existence of this harmony is inferred, because, in the large majority of "cases, the ontogeny leads to a typical result. Therefore we must assume that the conditions for the end result are given — the conditions are the harmony itself." Put somewhat less obscurely, if more crudely, we may express Driesch's idea by saying that the harmony that stands for a hen is given in the hen's egg.