Regeneration

by Thomas Hunt Morgan

If in the light of this discussion we turn to the phenomena of regeneration, we again find evidence showing that the germ-layer theory fails to apply in all cases. It has been pointed out that in lumbriculus, and in the naids, the new mesoderm is derived from the ectoderm, and does not come from the old mesodermal tissues. The mesoderm of the embryo in annelids is derived from one, and later from two, superficial cells of the blastula,[104] that push in about the time of gastrulation. They cannot, at this time, be referred to one layer rather than to the other. It cannot be affirmed, therefore, that in regeneration, the mesoderm arises from a different layer from that in the embryo, but neither can this be denied. The most important point in this connection is that the new mesoderm comes from the ectoderm that is already differentiated, and not from the mesodermal tissues. It is clear, however, that while the lining of the pharynx in the embryo is ectodermal, it is endodermal in the regenerated part.

It is true that these cases are very exceptional, and that generally the new organs come from similar organs in the old part, but one established exception is sufficient to show that the traditional conception of the germ-layers may be of little value, and since the hypothesis itself, out of which the idea in regard to regeneration from definite germ-layers has been formed, has been proven to be insufficient in other directions, the time is ripe to look for a more secure footing. It need hardly be added that the idea of a supposed necessity for an organ to arise from a definite germ-layer is so empty of all significance that we may well rejoice to be able to set it aside as a naïve view that has had its day. Furthermore, a new series of problems has arisen in connection with the experimental work to be described in a later chapter. If, as seems probable, the question of the germ-layers will be merged into the much broader question of the origin of the specification of the tissues, we can in the future more profitably direct our attention to the experimental evidence that bears on the latter question.

THE SUPPOSED REPETITION OF PHYLOGENETIC AND ONTOGENETIC PROCESSES IN REGENERATION

It has been claimed that at times ontogenetic, and even phylogenetic, processes are repeated during regeneration. Fraisse, for instance, who advocates this point of view, thinks that it has been too much neglected, and calls attention to several instances of what he believes to be cases in point. He thinks that Bülow is correct in his comparison between the method of development of the new tissue at the end of the tail in certain naids, and the method of gastrulation and formation of the mesoderm in the embryo. Later results have shown, however, that in several points Bülow’s observations are incorrect. The in-turning of ectoderm that Bülow compares with the process of gastrulation is connected with the formation of the ectodermal proctodæum, and is not comparable with the development of the endoderm in the embryo.

Götte also, as we have seen, cites a case of resemblance between the regeneration of the limbs of the salamander and their mode of embryonic development. He finds the resemblances less marked as the animal becomes older. The resemblance is, however, not very close and of a rather general sort, and since the same structures develop in both cases out of the same kind of substance, it is not surprising that there should be some resemblances in the processes. This evidence is counterbalanced by the mode of regeneration of the tail in the adult of certain forms, and in the regeneration of the lens of the eye from the iris.

Carrière finds that the eye of snails regenerates from the ectoderm in much the same way as the young eye develops. Granted that the eye is to come from the ectoderm in both cases, and that the same structure develops, it is not to be wondered at that the two processes have much in common.

The mistake, I think, is not in stating that the two processes are sometimes similar, or even identical, but in stating the matter as though the regenerative process repeats the embryonic method of development. If the same conditions prevail, then the same factors that bring about the embryonic development may be active in bringing about the regenerative processes. In fact, we should expect them to coincide oftener than appears to be the case, but this may be due to the conditions being different in the young and in the adult.

It has been claimed also that in some cases there is regenerated a structure like that possessed by the ancestors of the animal. The stock example of this process is Fritz Müller’s result on the regeneration of the claw of a shrimp, Atypoida protimirum.[105] Fraisse and Weismann and others have brought forward this case as demonstrative. The animal is said to regenerate a claw different from any of those in the typical form, and one that resembles the claw of another related genus, Carodina. The value of evidence of this sort is not above question. Przibram has shown in other crustacea that when a maxilliped is cut off a structure different in kind often regenerates, but that after several months the typical structure returns. Do we find here an ancestral organ that first appears, and then gives way to its more modern representative? If it resembled the maxilliped of any other crustacean, the evidence would, no doubt, be accepted by those who accept the evidence furnished by Müller. What then shall we say to the case, first discovered by Herbst, in which the eye of certain prawns being cut off, an antenna-like organ regenerates? Since these antennæ are similar to those possessed by the same animal, shall we assume that it once had antennæ in place of eyes?

Another comparison, that Fraisse has made, is worth quoting as showing how far credulity may be carried. In the regeneration of the tail of certain lizards pigment first appears in the ectoderm of the new part and then sinks deeper into the layers. Fraisse found a lizard on Capri in which the tail is pigmented throughout life, and although he did not know whether or not the pigment is in the skin he suggests that this lizard represents an ancestral condition, that is repeated by the regenerating tails of other forms.

Boulenger (’88) pointed out that the scales over the regenerated tail of several lizards have a different arrangement from that of the normal tail, and furthermore, the new arrangement is sometimes like that found in other species. He claims that this shows that such forms are related, even where no evidence of their relation is forthcoming. That the conditions in the new tail may be different from those in the normal tail is shown by the absence of a vertebral column, etc.; therefore that the scales also should have a new arrangement is not surprising, but the facts fail, I think, to show that there need be any genetic relation between the forms in question. That the conditions in the new tail might be like those in an ancestral form may be admitted, but this is very different from assuming that the results show a genetic relation actually to exist. The main point is that, even if the results should be nearly identical, it may be entirely misleading to infer that ancestral characters have reappeared.