Regeneration

by Thomas Hunt Morgan

Sachs has advocated a view which has many points of similarity to that of Bonnet, although, in reality, it is not a theory of pre-formation at all, but one of pure epigenesis. His idea rests on the view that the form of a plant, or of an animal, is the expression of the kind of material of which it is composed. Any change in its material leads to a corresponding change in the form of the new parts. Sachs holds that the idea of many morphologists, that there is for each organism a specific form that tends to express itself, and which controls the development of the organism, is a metaphysical idea that has no ground in science. For instance, Sachs thinks that the flower buds of a plant develop, not because of some innate, mystical force that causes the plant to complete its typical form, but because some substance is made in the leaves which, being carried into the growing region, becomes there a part of the material of that region, and from this new material a flower is formed. Simple and clear as this hypothesis appears to be at first sight, it will be found on more careful examination that it fails to account for some of the most characteristic phenomena of development and of regeneration. It may be granted at the outset that the presence of certain substances may undoubtedly influence the kind of growth of a new part; but, on the other hand, one of the most characteristic things of the organism is that it asserts its specific nature within quite a wide range of change, and, on the whole, resists the influence of other kinds of substances than those connected with its ordinary life. While Sachs looked no farther than the material substratum, and supposed that any change in this altered the form, there is, at present, sufficient evidence to show that it is the structure of the material that determines the most important changes that take place in it. This means, if we attempt to divest the statement of its somewhat metaphysical appearance, that the material of the organism is not simply a mixture of different kinds of materials, but a special kind of substance that has a definite structure of its own. This structure may, of course, be changed, but only by the addition of materials that the structure can take up as a part of itself. If the material does not become a part of the structure or organization, it is without effect on the form.[130] My meaning can, perhaps, best be illustrated by the method of regeneration of the tail of the fish from an oblique cut-surface. The growth of the new part is not determined by the kind or by the amount of the new material that is brought to the growing part, for, if it were, the new part would grow at an equal rate at every point; but the growth of the new part is regulated by the form of the tail of each particular kind of fish. The structure of the new part controls the growth of the material of the new part, and not the reverse. The only interpretation that can be given to this result is, I believe, that the new material assumes a definite structure, or what we may call an organization, and the subsequent changes are controlled by the kind of structure that is present; and since this structure has, as a whole, a definite form, we can state that the form controls the material, although the substitution of the word “form” for that of “the structure of the new material” may give the statement an unfortunate, metaphysical appearance.

In order to explain the regeneration of a piece of a plant, Sachs supposes that two substances are produced by the plant,—one a stem-(or leaf-) forming substance and the other a root-forming substance. If either of these substances combines with the protoplasm of any part, a stem or a root is produced from that part. When a piece of the stem is cut from a plant, these two substances accumulate, one at the distal end and the other at the proximal end of the piece, and their presence in these regions determines that new shoots develop at or near the apex, and new roots at the base. Sachs tried to show that the direction of the flow of these two substances is determined by the action of gravity,—the lighter substance flowing to the higher parts, and the heavier to the lower parts. We find here reproduced Bonnet’s idea of specific substances flowing in definite directions; but Sachs goes farther, and gives an explanation of the cause of the different directions taken by the two kinds of substances, viz. that it is due to the action of gravity. Vöchting has shown, as we have seen, after a thorough examination of the method of development of pieces of plants, that Sachs’s hypothesis fails to account for the results; and he shows also that an internal factor, which he calls the polarization, has the most important influence on the regeneration.

It is not difficult to show that there are many other cases to which the stuff hypothesis does not apply. If, as Bonnet attempted to show, the regeneration is due to different stuffs, there is no explanation to account for the flow in animals of head-forming stuffs forward and tail-forming stuffs backward. In animals that regenerate laterally as well as anteriorly and posteriorly, we should be obliged to assume side-forming stuffs as well as head-forming and tail-forming stuffs; and since the kind of structures that are produced at the side are different at each level, we should be obliged to assume that there are many kinds of lateral stuffs. If regeneration can take place in a dorsal and in a ventral direction, as, for example, when the dorsal and the anal fins of teleostean fishes regenerate, there must also be stuffs to account for their development. When regeneration takes place from an oblique surface, it must be supposed that two or more of these kinds of stuff are brought into action. The regeneration of just as much of the limb of the salamander as is cut off also offers difficulties for Sachs’s view. If we assume a leg-forming substance, it fails to account for the difference in the result at each level. If we assume that different substances come into play according to the amount of the leg that has been cut off, the hypothesis becomes as complicated as the facts that it pretends to explain.

A special case, to which the stuff hypothesis has been applied by Loeb and by Driesch, is that of tubularia, although the latter writer has used the hypothesis only to a limited extent as involving quantitative rather than qualitative results. There is present in the hydranth and stem of tubularia a red pigment in the form of granules in the endodermal cells. There is more of the red pigment in the stem near the hydranth than elsewhere. If a piece of the stem is cut off, it closes its cut-ends, and a circulation of fluid begins in the central cavity. In this fluid globules now appear that contain the red-pigment granules. The globules appear to be free endodermal cells, or parts of such, that have been set free in the central cavity. In the course of twenty-four hours the new hydranth begins to appear near one end of the stem, and in this region of the stem a much larger number of granules appear. A little later all the red granules disappear from the circulation.

Driesch has supposed that the red granules of the circulation become a part of the wall of the new hydranth. The disappearance of the red granules at this time from the circulation would seem to give color to this view. But, on the other hand, I have found evidence showing that this interpretation is incorrect. In the first place, the granules that disappear from the circulation can be found lying in a ball within the digestive tract of the newly formed hydranth; hence their disappearance can be accounted for, and we find that they are not, or at least in large part are not, absorbed into the forming hydranth.[131] In the second place, there is a great increase in the number of endodermal cells in the region in which the hydranth is about to appear, and the thickening that results takes place some time before the granules begin to disappear from the circulation. The new granules appear in the new endodermal cells, and are presumably formed by them. Again, the hydranth, that develops later at the distal end, appears when there are no granules in the circulating fluid, and yet the hydranth may contain as much red pigment as does the proximal one. Lastly, the development of very short pieces shows that at the time of the formation of the new hydranth there is an enormous increase in the number of red granules in the piece, for there are many more of them contained in the new hydranth than were present in the entire piece at the time of its removal.

Loeb has not referred to the red granules in the circulating fluid, but simply to the red pigment which is present in the walls of the piece. This is supposed to move forward into the hydranth region, and call forth the development of a new hydranth. A study of the number of the granules in the stem gives no support to this idea, and the method of formation of single and of double hydranths in short pieces shows that the increase in the number of granules in the hydranth-forming region is not due to migration, but to local formation.

That specific substances may have an influence on the growth of certain parts cannot be denied, but it appears that in general they play a very secondary rôle as compared with other factors that determine the form of the organism or the development of a part. Vöchting’s beautiful experiments (’86) on tuberous plants show that the presence of an excessive amount of food substances in the plant, brought about by the artificial removal of the natural storehouses for such material, may act on certain parts, such as the axial buds, or on the stem, and cause them to produce structures that they do not produce under ordinary circumstances. The axial buds become swollen and produce tuber-like bodies above ground, especially if the parts are enclosed so as to be in the dark, since the light retards the growth of tubers of all sorts. But it should not be overlooked that these buds and stems are structurally the same things as the tuberiferous stolons that have been removed, and hence the excess of material is stored up in them in the same way as it is under normal circumstances in the underground stems or stolons. The reaction is one normal to the plant, although it usually takes place in a different part.

The preceding hypotheses that have been advanced to account for the phenomena of regeneration, draw attention to some of the most fundamental problems of regeneration and, even in those cases in which the hypotheses have not given a satisfactory solution of the problems, some of them have served the good purpose, both of directing attention to important questions and of leading biologists to make experiments to test the new points of view. We should not underrate their value, even if they have sometimes failed to give a solution of problems, for they have been useful if only in eliminating certain possibilities, and this simplifies all future work. So long as an hypothesis is of a sort that it is within the range of observational and experimental test, it may be of service, even if it prove erroneous; for our advance through the tangled thread of phenomena is not only assisted by advances in the right direction, but all possibilities must be tested before we can be certain that we have discovered the whole truth. The value of a scientific hypothesis depends, it seems to me, first, on the possibility of testing it by direct observation, or by experiment; second, on whether it leads to advance; and, lastly, on its elimination of certain possibilities.

The experiments described in Chapters II, III, IV, have shown that there are many resemblances between the phenomena of growth and of regeneration. It has been pointed out that when it could be shown that certain external agents have a determining influence upon growth, these same agents have a similar effect upon regeneration. This also holds apparently for internal factors, although it is much more difficult to demonstrate that this is true. The presence of an abundance of food material in the tissues hastens regeneration in the same way that growth is more rapid in a well-fed organism. Food may, however, be looked upon rather as an external factor than as an internal one. An excellent example of an internal factor is found in the interrelations of the parts to each other. This is shown in the development of a piece of a plant in which the apical buds develop faster than the proximal ones, and it appears that, in some way, the development of the latter are held in check by the development of the apical ones. Another case is found in the development of the bilobed tail of certain fish in which particular regions are held in check, while others grow at the maximum rate.

It is a curious fact that while we can cite several kinds of external influences that affect the development and the regeneration of organisms, the only internal factors that have been discovered are the so-called polarity and this interrelation of the parts. Perhaps there should also be added the specific nature of certain parts, limiting the possibilities of new growth in these parts, and the presence of the nucleus as necessary for the growth and regeneration of the organism.