12 



REGENERA TION 



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 



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. 



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 re- 

 generate 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 ic not due 

 to the grafting, or to the anterior position of the posterior end, but to 



