Regeneration

by Thomas Hunt Morgan

Fig. 30.—Tubularia mesembryanthemum. A. Short piece with reduced hydranth-region. B. Piece from distal end of stalk producing a hydranth without a stalk (see [Fig. 27], D). C. Piece producing hydranth as outgrowth of end. C¹. Later stage of last. D. Short piece producing double proboscis (see [Fig. 28], E).

in all these cases the basal hydranth develops about twenty-four hours after the apical one. In the short pieces, however, the two ends develop at the same time, although the development of all the short pieces, whatever structures they may produce, whether single or double, is delayed, and the hydranths may not appear until after the long pieces have produced their basal hydranths. In these double structures both ends develop at the same time ([Fig. 30], D). If we suppose the influences that start the development of the piece begin first at the distal end, the region affected will lie so near to the proximal end of the piece that the development at this end may be hastened, and under these circumstances the region of new formation will be shared by the two hydranths. The factors that determine that a larger, partial structure is formed in preference to a smaller whole one will no doubt be found to be the same in these double structures and in the single ones.

THE INFLUENCE OF THE OLD PARTS ON THE NEW

One of the most striking and general facts connected with the phenomenon of regeneration is that the new part that is built up on the exposed surface is like the part removed. This suggests that an influence of some sort starts from the old part and changes the part immediately in contact with it into a structure that completes the old part in that region. We can imagine that the new part that has been changed in this way may act on the new part just beyond it, and so step by step the new part may be differentiated. It is not difficult to show that the phenomenon is really more complicated than this, and that other factors are also acting on the new part; but, nevertheless, that the old part has some such influence is probable. Under certain conditions, however, this influence may be counteracted by other factors, and something different from the part removed may be formed. One example of this sort has already been discussed, namely, that in which after the removal of much of the anterior end of the earthworm or of a planarian, only the distal end comes back. Another case is that in which something different from the part removed is regenerated. If the tip of the eye of the hermit-crab or of other crustaceans is cut off a new eye is regenerated, but if the eye-stalk is cut off near its base an antenna-like organ develops. Herbst has suggested that the presence of the ganglion at the end of the stalk accounts for the regeneration of a new eye, when only the tip of the stalk is cut off. In the absence of the ganglion at the cut-edge the stalk does not produce an eye, but an antenna, as is shown when the eye-stalk is cut off near the base. The factors that determine the development of an antenna instead of an eye have not been discovered. Przibram has shown that when the third maxilliped of portunas, carcinas, or of other crustaceans is cut off near the base, the new appendage that develops is different from the one removed, and resembles a leg in many ways, but if the animal is kept until it has moulted several times the appendage becomes more and more like the part removed. Another remarkable case has also been described by Przibram for Alpheus platyrrhynchus. In this decapod, the claws of the first pair of legs are different from each other, one being much larger than the other and having a different structure.[28] If the larger claw is thrown off at its breaking-joint, and the smaller one left intact, the latter at the next moult (or sometimes after two moults) changes into the characteristic larger claw and the newly regenerated claw is like the smaller one. If the experiment is repeated on this same animal, i.e. if the newly acquired large claw is removed, then at the next moult the smaller claw becomes the larger one and the new claw becomes the smaller one—the conditions now being the same once more as at the beginning. If both claws of an animal are thrown off at the same time, two new claws regenerate that are both of the same size, and each is a small copy of the claw that was removed. As yet no experiments have been made that show what factors regulate the development of each kind of claw.

Returning again to the question of the regeneration of parts similar to the ones removed, there are some interesting results that Peebles has obtained in the colonial hydroids, podocoryne and hydractinia. These colonies consist of three principal sorts of individuals: the nutritive, the reproductive, and the protective zooids. Peebles has found that if the stalks of these zooids are cut into pieces, each produces the same kind of zooid as was originally carried by that stalk. Pieces of the stem of the nutritive zooid produce new nutritive zooids at the anterior end of the piece, and sometimes also at the basal end. A similar statement may be made for each of the other kinds. Another method of regeneration sometimes takes place, when, for instance, a piece of the stalk of a nutritive individual is left undisturbed without being supplied with fresh water. It sends out root-like stolons instead of producing a new zooid. The stolons appear first at the ends of the piece, but may later also appear at several points along the piece. They make a delicate network, and the original piece may entirely disappear in the stolons. After several days new feeding zooids grow out at right angles to the stolon network. Pieces of the stalk of protective zooids may also produce stolons, but they spread less slowly, and the formation of new individuals was not observed. In one case a piece of a reproductive zooid made a short stolon, and from it arose a new individual that seemed to be a nutritive zooid. If the latter result proves to be true, we see that a piece may produce a new part that is of a different kind from that of which the piece itself was once a part, but this is brought about by the formation of a stolon that is itself one of the characteristic structures by means of which these colonial forms produce new nutritive zooids. In this case there is a return of the piece to a simpler form, the stolon, and, acting on this, the factors that produce nutritive zooids may bring about new nutritive zooids. The influence of the old structure is lost when the piece assumes a new character.

Another series of experiments gives an insight into an internal factor of regeneration that may prove, I think, to be one of some importance and help in interpreting certain phenomena. If the head-end of a planarian is cut off, the posterior piece split along the middle line, and one side cut off, just above the lower end of the longitudinal cut, as shown in [Fig. 31], A, it will be found that, if the long and the short sides are kept from uniting along the middle line, each half will produce a new head on its anterior surface ([Fig. 31], C). If the two halves grow together, and the anterior surface of the shorter piece becomes connected with the anterior surface of the longer piece by means of the new tissue that develops along the inner side of the latter ([Fig. 30], B), then a head appears only on the anterior half. The development of a head on the shorter half is prevented by the establishment of a connection with the new side. Sometimes an abortive attempt to produce a head is made, but the posterior surface fails to produce anything more than a pointed outgrowth. If we attempt to picture to ourselves how this influence of the new side on the posterior surface is brought about, we can, I think, most easily conceive the influence to be due to some kind of tension or pull of the new material which is of such a sort that it restrains the development of a head at a more posterior level. We can picture to ourselves the same kind of process taking place in the regeneration of the tail of a fish from an oblique surface. The maximum rate of growth is found over that part of the cut-surface that is nearer the base of the tail ([Fig. 40]). At all other points the growth is retarded, or held in check, and it can be shown that the suppression is connected with the formation of the typical form of the tail in the new part. If we cannot actually demonstrate at present that this is due to some sort of tension between the different parts which regulates the growth, we find, nevertheless, that it is by means of some such idea as this that we can form a clearer conception of how such a relation of the parts to each other is established. In a later chapter this subject will be dealt with more fully.

Fig. 31.—Planaria lugubris. A. Showing how worm was operated upon. B. A single head regenerated at anterior cross-cut. It was united by a line of new tissue along the side of the long half-piece with the new tissue at the anterior end of the short half-piece. The two half-pieces reunited along the middle line. C. Two heads regenerated, one from each half cross-cut. The two half-pieces were kept apart along the middle line.