Regeneration

by Thomas Hunt Morgan

The regeneration of the anterior end of the earthworm has been carefully worked out by Hescheler, and although on account of the greater complexity of the process the results are not so decisive as those just described, yet in many respects they are in agreement. In Hescheler’s experiments only four or five anterior segments were cut off. The closing of the cut-end is somewhat different from that in lumbriculus. A plug of cells soon forms over the end (Fig. 58, A). The new cells appear to be lymph cells. Although this mass of cells may be quite large, the cells do not seem to form later any of the organs in the new head. The presence of these cells makes it very difficult to work out the origin of the other cells that appear later. Owing to the absence of this lymph plug in lumbriculus and nais it is easier to follow in them the regenerative processes. In the midst of these lymph cells spindle-like cells soon appear whose origin is obscure, but Hescheler thinks it improbable that they are transformed lymph cells, although they are completely intermixed with the latter. The spindle-cells arrange themselves later in regular bands, that appear to be extensions of the longitudinal muscles. A few days after the operation, the lymph plug is covered over, beginning at the edge, by the ectoderm. The new ectodermal cells arise from the old ectoderm, and seem to extend over the lymph plug by a sort of migration process. Division of the cells does not occur at this time. These covering cells are at first all alike, the characteristic gland cells of the ectoderm being absent. The digestive tract withdraws somewhat from the outer cut-surface, and its end closes. The closed end abuts against the inner surface of the lymph plug. The next changes are initiated by the appearance of karyokinetic divisions in all the tissues of the new part, which lead to a rapid growth and elongation. Dividing cells are found in the new, as well as at the border of the old, ectoderm, where the new and the old parts are continuous. At this stage there appears in the lymph plug another kind of cell, that seems to arise, in part at least, from the ectoderm by an in-wandering of new cells. Other new cells may come from the edge of the old muscles, but it is not clear whether they come from a transformation of muscle cells, or from undifferentiated cells lying in the old muscles. In addition to these sources of new cells, it appears not improbable that cells may separate from the end of the digestive tract.

Nerve fibres push out from the end of the ventral nerve-cord into the new part, and groups of cells, often in process of division, appear in the old ganglia, even in those that lie a long distance from the anterior end. It is not improbable, Hescheler thinks, that new cells, as well as fibres, grow forward from the most anterior end of the nerve-cord into the new part. A mass of nerve cells and fibres appears in front of the old nerve-cord, and extends upwards and around the digestive tract, to meet over the anterior end of the latter in another mass of cells that have arisen from an early in-wandering of ectodermal cells. It is not improbable that the masses around the digestive tract (the commissures) and also the new ventral cord may also include cells that have had the same origin.

A tubular invagination of ectoderm is formed at this time at the anterior end. It meets the anterior end of the digestive tract; the two fuse, and the communication of the digestive tract with the outside is established. The pharynx develops from the anterior part of the digestive tract, which after Hescheler’s operation may contain some of the original ectodermal stomodæum, since only five of the anterior segments were cut off, and the embryonic stomodæum extends somewhat behind this region. In another experiment, carried out by Kroeber, somewhat more of the anterior end was removed, but the result was the same ([Fig. 59]), so that it is clear that the new pharynx may be formed from the old endoderm.

Fig. 59.—After Kroeber. Regeneration of anterior end of Allolobophora fœtida, after removal of six segments. The first stomodæal invagination had been destroyed. The new pharynx is developing from the endoderm.

Hescheler leaves several points still unsettled, more especially the origin of the cells that give rise to the new musculature, but it is almost impossible to make out their origin in this animal, owing to the presence of the lymph cells. Hescheler’s discovery that the cells of the lymph plug do not themselves, in all probability, contribute to the new part, is an important result, and shows that these seemingly undifferentiated cells do not possess the power of giving rise to the different kinds of new tissues. The in-wandering of cells into this solid plug from the ectoderm, and perhaps also from other sources, and their subsequent union to produce the definitive organs, is also a point of capital importance, especially as it puts us on our guard against a too ready acceptation of the view that all cells in a mass that have the same general and undifferentiated appearance have had a similar origin, and in showing that apparently indifferent cells may really carry with them into the new part those characters that determine their fate. Other cells, apparently equally undifferentiated, and lying in the same position, may have quite different possibilities.

In the vertebrates, the regeneration of the tail and limbs of amphibia and of the tail of lizards has been studied by a number of investigators. The regeneration of the tail of several urodeles and of the larva of the frog was investigated more fully by Fraisse (’95) and by Barfurth (’91). If we examine first the results of Fraisse’s study of the tail of urodeles, which have bony vertebræ, we find the following changes take place. The cut-surface is covered by the skin bending over the exposed part, accompanied by a migration of cells from the edge of the ectoderm. Only the unspecialized cells leave the old ectoderm to wander out over the cut-surface; gland cells and sense cells are entirely absent from the new ectoderm. These kinds of cells develop later out of the undifferentiated cells over the new part. The development of new vertebræ does not follow the embryonic method of development. In the embryo the endodermal notochord is first laid down, and around this and the nerve-cord mesodermal cells accumulate to form the skeletal tissue. Later the notochord is largely obliterated, as the vertebræ develop, pieces of it being left along the vertebral column. In the regeneration of the tail of the adult animal, the remnants of the old notochord (even if exposed by the cut) do not take any part in the formation of new tissue. In fact, there is no notochord formed at all. From the injured vertebræ, or at least from their covering of skeletal tissue, cells are proliferated, out of which a cartilaginous tube develops, enclosing the new nerve-cord, which is growing out from the cut-end of the old cord. In this tube centres of deposition of calcareous material are formed, and the new vertebræ are produced in this way. The new nerve-cord develops from the cut-end of the old cord, and more especially out of the cells of the lining epithelium of the canalis centralis. The new muscles develop from cells that arise from the old muscles.

In the tadpole of the frog the regeneration of the tail takes place essentially in the way just described for the adult urodele, except that, there being only a notochord in the tail, only a notochord is regenerated. According to Fraisse, the new notochord develops from cells that arise from the sheath of the old notochord, and not from the vacuolated cells of the notochord itself. The notochord cells are, he states, derived from the endoderm of the embryo,[93] while the sheath arises from the mesoderm; hence the newly regenerated notochord that arises from the sheath of the old one comes from a different germ-layer. Exception may be taken to this statement, because in the frog’s embryo the notochord develops from tissue that is at first perfectly continuous with the mesoderm, and, in fact, may be called mesoderm; also because it is probable, in the light of more recent research, that both the notochord and its sheath have exactly the same origin.

It is known that the tail of lizards breaks off generally at a definite region near the base, and that the break does not occur between the vertebræ, but in the middle of a vertebra—in some species the seventh caudal. The vertebræ are thicker at their ends than in the middle, and are firmly held together by intervertebral cartilages. The centres of the caudal vertebræ are the weakest links in the chain, or at least the place at which the vertebral column is most easily broken in response to the contraction of the tail-muscles.[94] Fraisse and others speak of this arrangement as an adaptation for breaking off the tail.