Fig. 5.—Hydra viridis. A. Normal hydra. Lines indicate where piece was cut out. B, 1-4. Changes in a piece of A, as seen from the side. C, 1-4. Same as seen from the end. D, E, F. Later stages of same piece, drawn to same scale.
In the earthworm also we find some interesting facts connected with the regeneration of the terminal pieces. If one, two, three, four, or five segments are cut from the anterior end, they will die without regenerating. Pieces that contain more segments, six to ten, for example, may remain alive for a month or longer, but do not regenerate ([Fig. 3], A, B). That this lack of power to regenerate at the posterior end is not due to the smallness of the piece can be shown by removing from a piece of five segments one or two of its anterior segments. These will be promptly regenerated. Another experiment has shown, however, that if these small pieces can be kept alive for a long time, and also supplied with nourishment, regeneration will take place at the posterior end. If, for instance, a small piece of eight or ten segments has its anterior three or four segments cut off, and is grafted by its anterior end to the anterior end of another worm, as shown in Fig. 3, F, the piece will begin, after several months, to regenerate at its exposed posterior end, but in the one instance in which this experiment has been successfully carried out, a new head, and not a tail, appeared on the exposed free end. The result is not due to the grafting, or to the anterior position of the posterior end, but to some peculiarity in the piece itself. We find the converse of this result in an experiment with the tail region of the earthworm, where the outcome is more clearly seen to be connected with the nature of the piece itself. If a piece less than half the length of the worm is cut off from the posterior end, there is generally formed from its anterior cut-surface, not a head, but another tail ([Fig. 2], I). The result is similar to that described by Bonnet for one of the fresh-water annelids. A parallel case to that of the head of the earthworm is found in one of the planarians. If the head of Planaria lugubris is cut off just behind the eyes ([Fig. 4], F), there is produced, at the posterior cut-edge of the head, a new head turned in the opposite direction, as shown in [Fig. 4], F¹.
REGENERATION BY TRANSFORMATION OF THE ENTIRE PIECE
Fig. 6.—A. Piece of Bipalium kewense. Middle pigment stripe injured at two points (see circles in A). B. Regeneration of same piece.
In the regeneration of some of the lower animals, the transformation of a piece into a new animal of smaller size is brought about by a change in form of the piece itself, rather than through the production of new material at the cut-ends. If a ring is cut from the body of hydra, as shown in [Fig. 5], A, the open ends of the ring are soon closed by the contraction of the sides of the piece, and in the course of a few hours the ring has become a hollow sphere; or, if the piece is longer, a closed cylinder. After a day or two, the piece begins to elongate, and four tentacles appear near one end ([Fig. 5], B, C, D). The piece continues to elongate until it forms a small polyp, having the typical proportions of length to breadth ([Fig. 5], E, F). It has changed into a new cylinder that is longer than the piece cut off, but correspondingly narrower. In this case there cannot be said to be a replacement of the missing parts, but rather, through the transformation of the old piece, the formation of a new whole. In planarians also the formation of a new worm from a piece involves a change in the form of the old part, as well as the addition of new material at the cut-end. If a cross-piece is cut out, as shown in [Fig. 4], D, new material appears at the ends, but the old piece also becomes narrower and longer ([Fig. 4], D¹-D⁴). If the old head is cut off, it produces new material at its posterior end ([Fig. 4], E, E¹), and also becomes smaller as the new part grows larger ([Fig. 4], E², E³). In a land planarian, Bipalium kewense, a piece is transformed into a new worm, as shown in [Fig. 6], A, B. In this case the old pigment stripes of the piece are carried directly over into the new worm, the piece elongating during the transformation.