Fig. 48.—After Rand. A. Head of Hydra cut off. After eight days. A¹. Same after thirteen days. Three tentacles misplaced. A². Same after eighteen days. A³. Same after twenty-one days. Misplaced tentacles absorbed. B. Anterior end of Hydra fusca, grafted upon side of body of another individual. Half an hour after operation. B¹. Same after four days. B². Same after thirty-eight days. B³. Same, foot-region after forty-nine days. B⁴. Same after separating. Fifty-second day.
by Rand. A part of one hydra is grafted upon the side of another one in the following way. A groove is scratched in a film of soft paraffine covering the bottom of a dish filled with water. Another groove is made at right angles to the first one, and opening into it. A hydra (the stock) is placed in the first groove, and a wound made in its side with a knife. Another hydra is cut in two, and one piece (the graft) placed in the other groove, and its cut-surface brought into contact with the wound in the side of the first individual. If the operation is successful the exposed surfaces of the two hydras quickly unite, and the combination may be taken out of the groove. If the piece grafted on the stock included about the anterior half of a hydra, a two-headed animal results, as shown in [Fig. 48], B. Although the graft has been united to the side of the stock, it soon assumes an apparently terminal position ([Fig. 48], B¹). This is due to the graft sharing with the anterior end of the stock the common basal portion of the stock. A slow process of separation of the two anterior ends now begins, brought about by a deepening of the angle between the halves ([Fig. 48], B²). This leads ultimately to a complete separation of the two individuals ([Fig. 48], B³, B⁴). Each may get a part of the original foot, or a new foot may arise on the graft as the division approaches the base.
In other experiments only a small part of the foot-end was cut from the animal that served as the graft. The long anterior piece was grafted as before upon the side of the stock. After the two had united, the graft was cut in two, leaving a part of the graft attached to the stock. The part regenerated tentacles, and in two cases subsequently separated from the stock as in the first experiment. In a third case the graft was absorbed by the stock as far as the circle of new tentacles, but its subsequent fate was not determined. In a fourth case the graft did not regenerate its tentacles, and was completely absorbed into the wall of the stock. The smaller the piece that is grafted on the stock the greater the chance that it will be absorbed, and furthermore short, broad rings are more likely to be absorbed than long, tubular pieces of the same volume.[80]
Rand’s results show in general that when hydras are grafted together they regain the typical form in one of two ways,—either by separation into two individuals, or by the absorption of the smaller into the larger component. In the former case the result is brought about in the same way as when the anterior end is partially split in two and the halves subsequently separate. When the graft is absorbed it is not clear whether the absorbed piece disappears or, as seems not improbable, forms a part of the wall of the stock.
It is important to notice the difference between lateral buds and lateral grafts. The buds separate in the course of four or five days by constricting at the base, but this never happens in lateral grafts. Rand has also made some experiments with buds. He cut off the outer oral end of a bud, and grafted it back upon the stock in a new place. It did not separate from the stock as does a bud, but by a slow process of division it was set free in the same way as are lateral grafts. The proximal end of the bud, which was left attached, developed tentacles at its free end, constricted at its base, and was set free. The separation was, however, somewhat delayed.
In another experiment a bud was split in two lengthwise, and the cut was extended so that the body of the parent was separated into two pieces. Twenty-four hours later it was found that each half-bud had closed in, and was much larger than when first cut. The half-bud, that was attached to the posterior end of the anterior piece, was constricting at its base, and subsequently it separated at its point of attachment. The other half of the bud, that had been left attached to the anterior end of the posterior piece, had swung around, so that its long axis corresponded to that of the posterior, parental piece. At first a slight constriction indicated the line of union of the two, but later this disappeared and a single hydra resulted. Whether the difference in the fate of the two half-buds is connected with their different polar relations to the parts of the parent, or is due to some other difference in the absorbing power of the anterior and posterior pieces, is not known.
Fig. 49.—After Peebles. A. Grafting in Tubularia mesenbryanthemum. A small piece of the stock taken from the region near the base, and grafted in a reversed direction on the oral end of a long piece. B. Same with distal tentacles in small piece, and proximal tentacles in large piece (modified from Peebles). C. Same. Formation of hydranth (original). D. Like A. Both pieces produce hydranths. E. Protrusion of hydranths of last. F. Piece of oral end cut off, turned around and grafted on oral end of long piece. A single hydranth produced. Distal tentacle from both components. G. A short piece from distal (oral) end of long piece cut off, and grafted by its proximal end to proximal end of the same long piece.