Regeneration

by Thomas Hunt Morgan

THE PART PLAYED BY THE “GERM-LAYERS” IN REGENERATION

Our examination of the origin of the tissues and organs in the new parts has shown that in most cases the old tissues give rise to the same kind of tissue in the new part; or in some other cases, as in the nervous system, the regenerating organs arise from the same “layer” as that from which they develop in the embryo. These facts have led many writers to state that the tissues and organs in the regenerated part arise from the same germ-layers as do the same parts in the embryo. It is supposed that ectoderm gives rise to ectoderm, and to those structures that arise from the ectoderm in the embryo, as, for instance, the nervous system, stomodæum, etc. The endoderm is supposed to give rise to endoderm, and to endodermal structures, and the mesoderm to mesoderm and its derivates. So fixed has this opinion become that it is not uncommon to find investigators proclaiming the triumphant success of their results, because they have been able to trace the organs in the regenerated part to the same germ-layers that give rise to these organs in the embryo. Before deciding as to the value of this point of view, let us examine briefly the foundations of the so-called germ-layer hypothesis.

The origin of this hypothesis goes back at least to 1759, when C. F. Wolff maintained his thesis that the digestive tract of the chick exists as a flat, leaf-like structure that subsequently rolls up into a tube. He thought it probable that other embryonic organs might arise in the same way. His views made at the time no impression on his contemporaries, and lay buried until 1812, when Meckel republished Wolff’s work in a German translation. Pander, in 1817, distinguished two layers in the early embryo, a serous and a mucous, and stated that later a third, vascular layer appears between the other two. Von Baer published in 1829 his celebrated memoir on the development of the chick, in which he made out two primary layers in the germ, the animal and the vegetative layer, and held that each of these separates into two to produce the four embryonic layers. Remak, in 1851-1855, gave a more precise description of the germ-layers, and stated that from the innermost layer, the epithelium and glandular cells of the digestive tract arise (including the lining of the glands that open into the digestive tract). From the outermost layer he showed that the integument and sense organs and the nervous system develop, and from the two middle layers develop the muscles, blood, excretory, and reproductive organs. By the term “germ-layers” was meant at this time only that the embryo is formed out of sheets.

Huxley in 1849 pointed out that a medusa is made up of two layers, an outer and an inner, and called attention to their possible equivalency to von Baer’s serous and mucous layers. This idea of a resemblance between the layers of an embryo and of an adult of a lower form furnished the starting-point for the more modern formulation of the germ-layer hypothesis. Kowalevsky’s work on the development of a number of the lower animals showed that there is present in many forms a two-layered stage, or gastrula, formed by an in-turning of the wall of the hollow blastula. In this way two germ-layers are established, an outer and an inner, that correspond to the ectoderm and to the lining of the digestive tract, or endoderm. While Kowalevsky’s work did much toward laying the foundation of the modern study of embryology, he himself indulged in very little of the sort of speculation that came into vogue a few years later. Kowalevsky’s discovery of the gastrula stage in the embryos of many different groups has been fully confirmed and extended, but the elaborate speculations that have been built up on this as a basis have gone far beyond the evidence, and, for a time, drew the attention of embryologists away from more important problems. Haeckel took a more extreme position than most of his contemporaries, and assumed that the gastrula stage that occurs in so many of the groups of metazoa corresponds to an ancestral, two-layered adult animal, the gastræa, from which all the higher forms have descended. The presence of the gastrula in the development was interpreted as a “repetition” of this ancestral adult stage. Thus the two primary layers are supposed to have an historical meaning.[100] Embryologists soon began a search for a similar mode of interpreting the middle germ-layer, or layers, which led, amongst other views, to the formulation of the “gut-pouch hypothesis.” From this point of view the body cavities, or cœlomes, are supposed to have been originally sac-like outgrowths from the digestive tract of an ancestral adult animal. Later, these cœlome sacs are supposed to have been shut off from connection with the digestive tract—their cavities becoming the body cavities, and their walls giving rise to the mesodermal organs. The formation of pouches from the walls of the archenteron of the embryo in several groups of animals has been interpreted as a repetition of the ancestral adult animal.

A comparison of the germ-layers in different forms very soon led to an attempt to “homologize” the layers in different animals. If the layers have had historically the same origin, or appear in the same way in the embryos, or give rise to the same organs, they are said to be homologous. In the absence of a knowledge of the first two of these conditions it is generally considered sufficient, if it can be shown that similar organs arise from a layer, to “homologize” that layer in the two forms. The study of embryology soon became a search for homologies. The results led to inextricable difficulties and innumerable contradictions until, a reaction setting in, many embryologists became sceptical in regard to the value of this entire method of study.

The results of a detailed study of the process of cleavage in a number of groups have helped, perhaps, to clear the way for a sounder conception. It has been found that the cleavage of the egg in members of the groups of annelids, mollusks, and turbellarians is extremely similar—so similar, in fact, that it seems hardly possible that they could be due to chance, especially as the series of cleavages is quite complicated. The discovery of these similarities led at once to comparison, and comparison to the establishment once more of homologies, and the homologies led again to contradictions, until at present scarcely any two workers agree as to a criterion of homology.[101] Leaving this question aside, however, and fixing our attention only on the similarity of the process of cleavage, we are justified, I think, in looking for an explanation of the similarity in some sort of an historical connection. We can eliminate, I think, without discussion the possibility of this type of cleavage representing an ancestral adult animal. So far as the question of descent enters the problem, we can infer with some degree of probability that the groups in question may have come from a common group in which the egg divided in much the same way as we find it dividing at the present time. As a formal hypothesis this view meets with no serious difficulty, since a chain of forms, or a continuous living substance, connects the present animals with those living in the past; and we may assume that the same factors peculiar to the egg of the ancestors are still present in the eggs of their descendants. This sort of explanation gives us no causal knowledge of the way in which the egg divides, nor does it preclude the possibility of new changes coming in that may entirely alter the form of the cleavage. Moreover, since we are dealing with a question of historical probability only, we cannot be certain that the same type of cleavage may not have arisen quite independently in each group.

The argument in favor of the gastrula stage also representing an ancestral larval stage may be admitted as a remote possibility, but on evidence even far less satisfactory than that for the similarities of cleavage being accounted for by a common descent. That this gastrula was ever an adult form we have no means of deciding, even as a matter of probability, and even if this could be made plausible it by no means follows that such an adult stage would become an embryonic stage of later forms. Consequently that part of the germ-layer theory that rests on such a supposed connection cannot be looked upon as much more than a fiction.

But even granting that there is an historical, embryonic[102] connection, its small importance for the scientific problems connected with embryonic development, and budding and regeneration has been shown by a number of recent discoveries, and nowhere more clearly than in the cases of the formation of new individuals by budding. As an example may be cited the method of development of the ascidian from the egg, and by means of buds. The work of Kowalevsky, Della Valle, Seeliger, and Van Beneden on the budding process of ascidians showed that there are some discrepancies between the bud development and the embryonic development. The more recent papers of Hjort, Oka, Pizon, Salensky, Lefevre, and others have shown very clearly that the germ-layer theory is inapplicable to the bud development in this group. The bud arises as a double-walled tube, or rather a tube within a tube, with a space between. The outer tube comes in all cases from the ectoderm of the animal; the inner tube has a different origin in different species. In perophora, didemnum, and clavellina, the inner tube comes from endoderm; in botryllus it arises from the ectoderm of the larval peribranchial or atrial cavity. In all these forms the inner tube gives rise to the new pharyngeal cavity of the bud, while this same cavity comes from the endoderm of the archenteron of the embryo. In the bud embryo the peribranchial space is also derived from the inner tube; hence it is endodermal in the first series, and ectodermal in botryllus. In the egg embryo it is ectodermal. In regard to the development of the nervous system there is some difference of opinion. A number of investigators have found that the new brain arises from the outer part of the inner or branchial tube, which has in most cases an endodermal origin. Seeliger and Lefevre believe the nervous system to arise from mesodermal cells that lie between the two tubes. It appears, nevertheless, that in several forms the brain really comes from the inner tube, which also gives rise to the branchial sac. Therefore, in those cases in which the inner tube is endodermal the brain has the same origin, and in the case in which the inner tube is ectodermal, the brain is ectodermal, but the pharyngeal sac has also an ectodermal origin. There is obviously no definite relation between the origin of these structures in the bud and in the egg embryo.

A similar difficulty is met with in the Bryozoa in regard to the development of the egg embryo and the bud embryo.

Braem, who has made a critical examination of the germ-layer theory,[103] has found it impossible to give a morphological definition of a germ-layer, and has adopted a physiological criterion. He thinks that in whatever way a germ-layer arises, whether by folding, or by delamination, etc., it exists independently of its method or place of origin. A layer is not endodermal because it forms the inner wall of a gastrula, but it is endodermal because it develops into the digestive tract. The germ-layers of different forms are only similarly placed, but whether they are homologous will depend on other things. On this view the inner tube of the ascidian bud that gives rise to both digestive tract and to the nervous system is simply an indifferent layer until it gives rise to these structures. Its cells may be looked upon as indifferent, as are those of the blastula. Thus the difficulty of the morphologist is not solved, but the knot is cut. For Braem the germ-layers are convenient terms, since he rejects any historical significance that they may have, and it is just this side of the question that the morphologist has attempted to work out. While the evidence shows that the germ-layers cannot have any such final attributes as embryologists have attempted to assign to them, and that Braem has called attention to the real and important problems connected with the study of development, yet it may still be admitted without endangering the newer point of view, that there may be also an historical question in connection with the germ-layers, if not in the sense of a repetition of an ancestral adult gastræa, yet in the sense that similarity in embryonic development may in some cases find its historical explanation in a common descent.