GENERAL INTRODUCTION II 



relation to the animal as a whole, but in a longitudinal direction 

 in regard to the limb itself. Lateral regeneration of the limb would 

 take place if the limb was split lengthwise into two parts and one of 

 the parts removed. If the entire salamander were cut in two length- 

 wise, each half would most certainly die without regeneration, if for 

 no other reason than that the integrity of the median organs is 

 necessary for the life of the different parts. If, however, a planarian 

 is cut lengthwise into a right and left half, each piece will complete 

 itself laterally and make a new worm (Fig. 13^, AD). Even a narrow 

 piece cut from the side will produce a new worm by regenerating 

 laterally, as shown in Fig. 19, a, b, c. In hydra, also, a half-longi- 

 tudinal piece produces a new animal, but in this case not by the 

 addition of new material at the side, but by the cut-edges meeting 

 to make a tube of smaller diameter. Subsequently the piece changes 

 its form into that characteristic of hydra. 



REGENERATION OF TERMINAL PORTIONS OF THE BODY 



In most of the preceding examples the behavior of the larger piece 

 of the two that result from the operation has been described ; but there 

 are some important facts in connection with the regeneration of the 

 smaller end-pieces. The leg, or the tail, that has been cut from the 

 salamander soon dies without regenerating. The life of the leg can 

 be maintained only when the part is supplied with certain substances 

 from the body of the animal. It does not follow, of course, that, 

 could the leg or the tail be kept alive, they would regenerate a 

 salamander. In fact, there is evidence to show, in the tail at least, 

 that, although it may regenerate a structure at its anterior end, the 

 structure is not a salamander, but something else. This has been 

 definitely shown in certain experiments with the tail of the tadpole. 

 It is possible to graft the tail of one tadpole in a reversed position, 

 i.e. with its anterior end free, on the tail of another tadpole (Fig. 54, 

 A-D\ or even on other parts of the body. Regeneration takes place 

 from the free end, i.e. from the proximal end of the grafted tail. 

 The new structure resembles a tail, and not a tadpole. If it be 

 objected that the experiment is not conclusive because of the presence 

 of the old tail, or of the use of the newly developing part, the objec- 

 tion can be met by another experiment. If, as shown in Fig. 56, A, 

 a triangular piece is cut out of the base of the tail of a young tadpole, 

 the cut being made so deep that the nerve-cord and notochord are 

 cut in two, there develops from the proximal end of the tail a new 

 tail-like structure that is turned forward, or sometimes laterally. In 

 this case the objections to the former experiment do not apply, and 

 the same sort of a structure, namely, a tail, is produced. 



