Regeneration

by Thomas Hunt Morgan

It is more difficult to unite pieces of different species of worms, yet Joest has succeeded also in making combinations of this sort. One union between the anterior end of Lumbricus rubellus and the posterior end of Allolobophora terrestris was permanent, and the new worm reacted as a single individual, and lived for eight months. Each piece retained its specific characters, and showed no influence of the other component. By means of a similar experiment we have a way of finding out if one component can influence regeneration taking place from the other piece. Although Joest made only a few observations of this sort, the results show that no such influence is manifested.

Fig. 53.—After Joest. A. Small piece of Allolobophora terrestris from posterior end grafted upon anterior end of another individual. Oral end free. Four weeks after grafting eight new segments formed. B. Same fourteen days later. A new part of thirty-seven segments had appeared at end of former eight segments. C. A piece of the body wall of Allolobophora terrestris grafted upon the cut-end (anterior) of Lumbricus rubellus. Two months later, as shown in figure, a head had grown on major component. D. Anterior and posterior ends of A. terrestris united to make a “short worm.” E. A piece of body wall of A. cyanea grafted on side of body of Lumbricus rubellus. F. Piece of L. rubellus grafted on side of body of another individual to produce a double-tailed worm.

By means of grafting it is possible to keep alive small pieces of a worm that would otherwise perish. For instance, pieces of a worm containing only three segments are not capable of independent existence, except for a short time, and even pieces of from four to eight segments die in most cases. It is not possible to unite small pieces of this size directly upon larger pieces, since they will die, ordinarily, as a result of the operation, but larger pieces can be united and then after union has been effected, one of them may be cut off near the place of union. The same result is sometimes brought about accidentally by the worms themselves pulling apart and leaving a small piece of one component attached to the other. Joest found that in several cases these small, attached pieces regenerated. In one case, after two long pieces had pulled apart, a small piece, left by one of the two, regenerated a single new segment with a mouth at its end. In another case, after one of the components had been cut off, leaving two segments attached, a new part of seven segments regenerated.[83] Especially interesting is the case in which two individuals (A. terrestris) had been united to form a long worm. The anterior component extended to within two centimetres of the anus; the posterior piece had had the first four segments removed. Three days later the anterior piece was cut off three segments in front of the region of union. About a month later a small part of eight segments had regenerated from the cut-end ([Fig. 53], A). Fifteen days later another new part of thirty-seven segments developed at the end of the first new part ([Fig. 53], B). Joest speaks of the first eight segments as a head, and the second simply as a regenerative product. There can be little doubt, I think, that both parts represent a heteromorphic tail. The region from which the regeneration took place would make this interpretation highly probable, and Joest’s figures also indicate that the structure is a tail. The result is very interesting, if my interpretation is correct, as it shows that the major component did not influence the kind of regeneration, although the surface of regeneration was separated by only three tail-segments from the anterior end of the major component.

In another experiment a long animal was made by uniting Lumbricus rubellus (whose posterior third had been cut off) and Allolobophora terrestris (whose first six segments had been cut off). Four days later the two components had torn apart, but a small piece of the anterior worm remained attached to the anterior end of the posterior component. The small piece consisted of the dorsal part of two and a half segments without any ventral part, so that the anterior end of the posterior component was partially exposed. The small piece of lumbricus was much lighter in color, and this difference made it easy to distinguish between the two. In less than a month the small transplanted piece had replaced its missing ventral part, so that the entire anterior surface of the larger component was covered over. The small piece, in addition to regenerating its ventral part of four segments, had also begun to make new segments. After a month and a half six new segments were present ([Fig. 52], E), with a mouth at the anterior end.[84] Even after ten months the color of the small piece was strikingly different from that of the major component. The new head had the typical red-brown color of L. rubellus, that forms a strong contrast to the grayish blue color of A. terrestris.[85] The result shows that the color of the regenerated part has not been influenced by that of the posterior component, and this is all the more interesting, as Joest points out, because the small piece that was left after the worms pulled apart was too small to have lived independently for any length of time, and must have derived all its nourishment from the larger piece.

In other experiments pieces of one species were cut from the side of the body and grafted upon the cut-surface of the anterior end (or elsewhere) of another species. In one of these experiments a piece from the side of A. terrestris, that extended over five or six segments, was sewed upon the anterior cut-surface of L. rubellus (from which the anterior five segments had been removed). In about a month new tissue appeared on the ventral side between the two pieces, and a little later a complete head developed, whose dorsal side was made up of the small piece ([Fig. 53], C). The grafted piece was dark, and the new, regenerated part light in color and continuous with the brown color of L. rubellus, from which the new part had arisen. It is important to notice that the four segments of the graft are completed by four segments of the new part. After three months the new part had assumed the red-brown color of L. rubellus. The color of the grafted piece had not changed. We see in this case that even the presence of a part of another worm in a regenerating region does not have any influence, at least so far as color is concerned, on the new part, even though its segments supplement some of those of the new part. The new tissue seems to have come entirely from the major component, and to have carried over the color characteristics of the old part.

It has been shown that when two posterior pieces are united by their anterior ends the combination must sooner or later die, since it has no way of procuring food. The question arises: What will happen if one of the two components is cut in two near the place of union? Will a head then develop on the exposed aboral surface, because a head is needed to adapt the worm to its surroundings, or possibly, if it occurred, because the major component exerts some sort of influence on the short, attached piece, as happens in hydra and in tubularia? Both Joest and I carried out an experiment of this sort, and found that a tail and not a head regenerated, as shown in [Fig. 16], F. The experiment is, however, insufficient to answer the question, since the region in which the second cut was made is a region from which only a tail (and not a head) arises, even when the oral end of a piece is exposed. In order to avoid this difficulty I carried out another experiment. Two worms had the first five or six segments cut off and the exposed anterior ends of the worms united, as shown in [Fig. 16], D. Then one of the components was cut off, leaving three or four segments attached to the anterior end of the other component. Although regeneration began in one case, it did not go far enough to show what sort of a structure had developed, but Hazen, who took up the same experiment, succeeded in one case in obtaining a definite result. At the exposed aboral end of the small piece a head and not a tail developed ([Fig. 16], E). At first sight it may appear that the result shows the influence of the major component on the small piece, causing it to produce a head and not a tail at its aboral end, but I think that this conclusion would be erroneous, because it seems much more probable that we have here a case of heteromorphosis, similar to that in Planaria lugubris, and that the result depends entirely on the action of the smaller component. It is hardly possible to demonstrate that this is the correct interpretation, since if a small piece of this size is isolated it dies before it regenerates. The result is paralleled, however, by the regeneration of a tail at the anterior surface of a posterior piece.

The process of grafting has long been practised with plants, but the experiments were made more for practical purposes than to study the theoretical problems involved. Vöchting has, however, carried out a large number of well-planned experiments. He finds that a stem can be grafted upon a root, and a root upon a stem, a leaf upon a stem or upon a root. Even an entire plant can be grafted upon another. The results show, however, in general, that, whatever the new position may be, the graft retains its morphological characters—a shoot remains a shoot, a root is always a root, and a leaf a leaf. Vöchting concludes that there is in the plant no principle or organization that conditions an unchangeable arrangement of the main organs. “The inherited order of the parts, acquired apparently on physiological grounds, may be altered by the experimentator; it is possible for him to change the position of the building blocks within a wide range without endangering the life of the whole.” “It is essential, however, for the success of the experiment that the grafted parts, or tissues, retain their normal orientation. If this condition is not fulfilled there may take place, it is true, a union of the parts, but sooner or later disturbances set in.” Vöchting transplanted pieces in abnormal positions, sometimes reversing the long axis of the grafted piece, sometimes the radial axes, and sometimes both together. In some cases this led to the formation of swellings that interfered with the nourishment but carried with it no further consequences. In other cases the changes went so far that the vital processes were interfered with. At times an incomplete union took place between the parts; at others, even though the first union was perfect, death later ensued.

On the other hand, when similar pieces were grafted with their original orientation, a perfect union took place and the piece became a part of the stock. The results establish, Vöchting claims, that every part and every portion of a part has a polar orientation in one direction, and furthermore, in a body having a radially symmetrical form, there is also a radial polarization; that is, the inner side of each part is different from the outer side of the same surface, even though no such difference is apparent to us. The properties of the tissue-complex rest, in the last analysis, on that of the cells; the properties of the whole being only the sum total of the properties of its elements, so that we may say that every living cell of the root is polarized, not only longitudinally, but also radially; each has a different apical and root pole, a different anterior and posterior pole, and also right and left polar relations. These results, deduced from the experiments in grafting, lead Vöchting to formulate the following rule: “Like poles repel, unlike poles attract.” This rule is the same as the law of the magnet. In fact, Vöchting states that the root and the stem relations show a remarkable resemblance, despite many differences, to a magnet. If the magnet is broken into pieces it may be reunited by bringing unlike poles together, but not by uniting like poles; the same statement holds for the root and the stem.