GENERAL INTRODUCTION 13 



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, /). 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 pos- 

 terior cut-edge of the head, a new head turned in the opposite direc- 

 tion, as shown in Fig. 4, F 1 . 



REGENERATION BY TRANSFORMATION OF THE ENTIRE PIECE 



In the regeneration of some of the lower animals, the transforma- 

 tion 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 con- 

 tinues to elongate until it forms a small 

 polyp, having the typical proportions of 

 length to breadth (Fig. 5, E, F). It has 

 cjianged 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, D l -D*\ If the old hi 

 it produces new material at its posterior end 

 (Fig. 4, E, E 1 ), and also becomes smaller 



it; rut off FlG - 6 -~ A - Piece of 



>rl kewetue. Middle pigment 



stripe injured at two points 

 (see circles in A). .#. Regen- 

 eration of same piece. 



