Regeneration

by Thomas Hunt Morgan

Tubularia is not so well suited as hydra to show the influence of grafting on the united parts, since pieces of tubularia produce hydranths, both at the oral and aboral ends, although the latter hydranths take longer to develop. Peebles has shown, nevertheless, that grafting has an influence on the behavior of a piece. In order to show that the polarity of a small piece could be affected by a larger piece, the following experiment was carried out. After cutting off the old hydranth from the end of a stem, a short piece was then cut from the distal end of the same stem, turned around, and its oral end brought in contact with the oral end of the original piece, as indicated in Fig. 49, F. The two pieces, being held together for a few minutes, stuck together and subsequently united perfectly. From eighty-eight pieces united in this way the following results were obtained. Thirty-six formed a single hydranth at the end at which the grafting had been made. The distal row of tentacles appeared in the smaller reversed component, the proximal row in the larger piece ([Fig. 49], B). The new hydranth pushed out later through the perisarc of the smaller piece ([Fig. 49], C). In this experiment the smaller component was shorter than the average length of the hydranth-forming region. In two cases, in which the smaller component was larger, both circles of tentacles appeared in this piece. In six of the experiments the tips of the proximal tentacles arose from a part of the wall of the smaller piece, hence these tentacles had a double origin ([Fig. 49], F). In five of the unions the smaller as well as the larger component produced a hydranth; the two were stuck together by their oral ends ([Fig. 49], D, E). The remaining four unions gave somewhat different results. In three of these the smaller piece produced only a part of a hydranth that remained sticking to the end of the hydranth formed by the larger component. In the thirty-six cases in which the minor component took part in the formation of the single hydranth, the influence of the larger component was shown not only in reversing the polarity of the smaller component, although this might in part be accounted for by the closing of the oral end of the smaller piece, but also in the time of development, since the hydranth appeared sooner than does the aboral hydranth and at the same time as does the oral hydranth.

In another series of experiments, a short piece was cut from the basal end of a long piece (three to four centimetres) and brought forward and grafted in a reversed position on the anterior end of the same long piece ([Fig. 49], A). Of five unions of this sort, one produced a hydranth in each component, neither being reversed. Another of the pieces produced a hydranth partly out of each component (and at the same time another at the aboral end of the large piece). The other two pieces produced a single hydranth, a part of which came from the minor component and appeared before the aboral hydranth on the aboral end of the larger piece. This last result shows that the small piece from the basal end has been affected by the oral end in such a way that it develops more rapidly than it would have done had it remained a part of the basal end.

In a third series of experiments a short piece (about a half of a millimetre) was cut from the anterior end of a long piece (one and five-tenths to two centimetres) and grafted in a reversed position on the posterior end of the same long piece ([Fig. 49], G). In four cases a hydranth developed only at the oral end of the long piece and none from the aboral end or from the short piece. Eight unions produced, however, in the region of the graft, a hydranth formed partly by each component. Later another hydranth developed at the oral end of the larger piece. The latter results are not convincing, but they may show that the small piece has hastened the development of the hydranth at the aboral end.

Peebles has also made some experiments in grafting pieces of different members of the colonies of hydractinia and podocoryne. The colony of the former is made up of three different kinds of individuals: the nutritive, the reproductive, and the protective hydroids. A series of preliminary experiments showed that if these individuals are cut into a number of pieces each piece regenerates the same kind of individual as that of which it had been a part. It was also observed that if pieces of the nutritive individuals were allowed to remain quietly on the bottom of the dish they sent out branching stolons, which stuck to the bottom of the dish, and from these stolons there arose later nutritive hydranths that stood at right angles to the surface. When pieces of the same kind of individuals are grafted together, the results are essentially the same as with tubularia. If pieces of different kinds of individuals are united, the opportunity is given of testing the possible influence of one kind on the other. Peebles united a nutritive and a protective polyp by the cut, aboral ends ([Fig. 46], E), and after they had grown together one of the polyps was cut off near the region of union, so that a small piece of a nutritive polyp was left attached to a protective polyp. When the piece of the nutritive polyp regenerated, it made a new nutritive polyp. The influence of the protective polyp was not apparent. If a nutritive and a reproductive polyp are united in the same way, and the latter cut in two near the line of union, a new reproductive polyp develops from the piece left attached to the nutritive polyp. Again there is shown no influence of the one on the other kind of polyp.

Hargitt has also made a number of grafting experiments on other hydroids. His most interesting results are those in which parts of two medusæ were united by holding their cut-surfaces together by means of bristles passing through the individuals. Hargitt also finds that while in certain hydroids it is possible to bring about a union of oral with oral end, or aboral with aboral, or oral with aboral end of the same species,[81] yet a permanent union between different species cannot be brought about. These results are in agreement with those of a number of writers who have recorded the difficulty or impossibility of uniting parts of different species of hydra. In a few instances it has been possible to unite temporarily a piece of a brown hydra with a piece of a green one,—as I have also seen accomplished,—yet the pieces subsequently separate. Wetzel succeeded in obtaining better results with two species of brown hydras, Hydra fusca and Hydra grisea. In one experiment the head of Hydra grisea was grafted on the body (from which the head had been cut off) of Hydra fusca. After five hours the pieces seemed to have united. Later a constriction appeared at the place of union, and the head-piece produced a foot near the line of union, and the posterior piece produced a circle of tentacles at its anterior end. Eight days later, when the animal was being killed, it fell apart into two pieces. It was observed that during the period of union a stimulus to one piece was not carried over to the other. Wetzel’s results seem to show that pieces of these two species of hydra unite at first, when brought together, as perfectly as do pieces of the same species, but the union never becomes permanent, a constriction appearing later at the line of union, and the pieces separating in this region. These results indicate, it seems to me, that the factors that bring about the first union are different from those that make the grafted pieces one organic whole. Other results indicate that the union of oral to oral end, or aboral to aboral end, while at first as perfect as between unlike surfaces, nevertheless is less permanent than when unlike surfaces are united; at least, subsequent regeneration is more likely to occur in the former than in the latter, and after this occurs the separation of the individuals often takes place. It seems, moreover, not improbable that a more permanent union results when similar regions are united by unlike surfaces, than when the union is at different levels. If, for instance, the anterior half of one hydra is united to the posterior half of another individual, the union is generally permanent; but if one or both of the pieces are longer than half the length, so that a “long animal” results, new tentacles are more often formed at the oral end of one component, and the parts subsequently separate. It may be that, at present, the data are insufficient to establish this general rule, and no doubt other modifying influences must be also taken into account; but it is important that attention should be drawn to this side of the subject.

Grafting experiments in planarians have so far been carried out in only the two cases which I have described. In one of these the anterior ends of two short pieces of Bipalium kewense were united ([Fig. 50], A). Neither piece produced a head at the region of union. Later the pieces were cut apart by an oblique cut that passed across the line of union ([Fig. 50], C), so that each piece retained at its most anterior end (at one side) a piece of the other individual in a reversed position. A head developed at the anterior (and lateral) end of each piece, in such a way that a part at least of the small reversed piece was contained in the new head ([Fig. 50], D). In the other case two pieces of bipalium were united by their posterior cut-surfaces. Each piece produced a new head at its free end, and the pieces greatly elongated, but remained sticking together ([Fig. 51]).

Fig. 50.—A. Two pieces of Bipalium kewense united by anterior ends. B, C. Later stages of same. Line in C indicates how pieces were cut apart. D. Two worms produced by these pieces. All drawn to scale.