Regeneration

by Thomas Hunt Morgan

THE CLOSING IN OF CUT-EDGES

One of the most familiar changes that takes place when a cut-edge is exposed involves the rapid covering over of the exposed tissues. This takes place from the margin of the wound, and a layer of cells, usually the ectoderm at first, covers the surface. The closing in is brought about in many forms by the contraction of the muscles of the outer wall of the body. This seems to be the case in the earthworm and in the planarian, as well as in other animals, such for instance as the starfish, holothurian, etc. But in addition to this purely muscular contraction another process takes place, that is less conspicuous in forms in which the muscles bring about the first closing, but which is evident in forms in which the muscles are absent or little developed. I am able to cite two striking cases that have come under my own observation. When a piece is cut from the stem of tubularia, the ends close in twenty minutes to half an hour. The body wall, the cœnosarc, composed of the two layers of ectoderm and endoderm, withdraws a little from the cut-edge of the outer hard tube, or perisarc, that covers the stem, and then begins to draw across the open end. A perfectly smooth, clean edge is formed that advances from all points to the centre, where the final closing takes place. The closing is not due to an arching over of the cœnosarc, but the thin plate is formed standing nearly at right angles to the outer tube. This plate is composed of two layers of cells, of which there are a number of rows arranged concentrically between the centre and the outer edge. In the absence of muscle-fibres in the stem, the result cannot be due to a muscular contraction, and even if short fibres existed the transportation of cells entirely across the open end would speak against this interpretation.[30] Since the closing over takes place without any support, we cannot suppose the process to be due to any sort of cytotropic effect. The closing takes place equally well in diluted sea water and in stronger solutions. The method of withdrawal of the cells, as best seen when longitudinal pieces are studied, resembles very much the withdrawal or contraction of protoplasmic processes in the protozoa, and so far as one can judge from resemblances of this sort, the two processes appear to be the same.

This closing in of the cut-surface, while a preliminary step in the process of regeneration, cannot, I think, be regarded as a part of the regeneration in a strict sense. That the two processes are not dependent on the same internal factors is shown by the following experiments: If a bunch of tubularia is kept in an aquarium, it will produce new heads two or three times and then cease, and if after the last-formed heads have died, pieces of the stem are cut off, they close as readily as do pieces from fresh hydroids. Moreover, at certain times of year the species Tubularia (Parypha) crocea lose their heads, and only the stalks remain. Pieces of these stalks will not regenerate new heads, at this time, although they close in as quickly as do pieces at other times of the year when the heads are present and when new ones regenerate.

Another equally good illustration of what seems to be the same phenomenon is found in the closing in of wounded surfaces in the young tadpole embryos. If embryos are taken from the jelly membranes, or even after they have been set free, and cut in half, each piece quickly covers over the wounded surface by means of the ectodermal cells. A much more striking illustration of this closing over in the young tadpole is obtained by cutting, with a pair of small scissors, a large piece from the side. The area may be a fourth or more of the entire side, and yet it may be closed over in an amazingly short time. Half an hour or an hour often suffices to cover a large exposed surface. In this case also the wound is covered not by individual cells wandering over the exposed surface, but by a steady advance of the smooth edge of the ectoderm toward a central point. The process is so similar to that which takes place in tubularia that little doubt can remain as to the two being due to the same factors. As there are no muscle fibres present in the part of the frog’s embryo from which the piece is cut off, the result cannot be due to muscular contraction, but appears to be a contractile phenomenon similar to that in tubularia. Even the small piece that is cut from the side of the body shows the same phenomenon. At first it suddenly bends outwards owing to some physical difference between the inner and the outer parts of the piece. Then the edges thicken, bend in, and begin their advance over the inner tissues. The process is seldom completed, since there appears to be a limit to which the ectoderm can be stretched as the edges advance. A most striking phenomenon both in pieces of tubularia and of the frog’s embryo is the entire absence of dead material at the wounded surface. No sooner is the operation performed than the advance begins; and there is not a trace of dying cells or parts of cells to be seen.

CHAPTER IV
REGENERATION IN PLANTS

The series of experiments that Vöchting has carried out on the regeneration of the higher plants are so much more complete than all previous experiments, and his analysis of the problems concerning the factors that influence regeneration is so much more exact than any other attempts in this direction, that we may profitably confine our attention largely to his results. Many of his experiments were made with young twigs or shoots of the willow (salix), which, after the removal of the leaves, were suspended in a glass jar containing air saturated with water. Under these circumstances the pieces produced new shoots from the buds (leaf-buds) that are present near the point at which the leaves were attached, and new roots, in part from root-buds, that are also present on the stem.

If the piece is suspended in a vertical position with its apex upward ([Fig. 32], A), small swellings appear after three or four days near the lower, i.e. the basal, end of the piece. These break through quickly and grow out as roots. If a leaf-bud is present near the basal end of the piece, the first roots appear at the side of or under this; later others appear around the same region. The first roots to appear under these conditions come from pre-formed root rudiments, the others are, in part at least, new, adventitious roots. If the lower end of the cut is made through the lower part of a long internode, i.e. just above a bud, the roots appear as a rule only near the cut-end, and few if any of the roots develop at the first bud above this region. In many cases there is formed over the basal cut-surface, in the region of the cambium, a thickening, or callus, and not infrequently from this also one or more roots may develop. The direction taken by the new roots is variable, being sometimes downward, sometimes more or less nearly at right angles to the stem.

While these changes have been taking place at the base, the leaf-buds at the apical end have begun to develop. One, two, three, four, or even five of the higher buds begin to elongate, the number and extent of development depending on the length of the piece. The topmost or apical bud grows fastest, and the others grow in the order of their position. In the region below the lowest bud that develops there may be one or more buds that do not grow; but if the piece is cut in two just above these buds, they will then grow out.