Xo. 493] EXPERIMENTS IN GRAFTING ( J 



regenerate can be shown by cutting' from the side of the 

 gastrocnemius a square piece of muscle tissue. In the 

 course of a few months I have found that the muscle 

 regains its size, which seems to be due, in part at least, 

 to the formation of new muscle, although hypertrophy 

 of the remaining fibers may also assist in the enlarge- 

 ment. 



There is some indication that the delay in the forma- 

 tion of the new leg in the frog is due to conditions exist- 

 ing in the bones or muscles and, as I have pointed out, 1 

 it is significant to find that in the vertebrates the loss of 

 power to regenerate a limb appears where cartilage has 

 been changed to bone. The result is not however due 

 directly to the ossification, since the new material is 

 derived from the periosteum and not from differentiated 

 tissue. 



Especially interesting is the evidence showing that the 

 introduction of material, itself capable of regeneration 

 (as when the tail-tissue of the lizard is introduced into 

 the leg-pocket) does not incite the leg to regenerate. If 

 the process of regeneration is due to some enzyme, or 

 oilier substance of this nature, that arises in an injured 

 region, and whose presence incites the new growth, we 

 might hope by introducing pieces of material capable 

 of forming such substances to incite regeneration, but 

 no such result followed. It would be unwise to lay too 

 much weight on negative evidence of this kind, but the 

 results as they stand indicate, perhaps, with some proba- 

 bility, that the primary cause of regeneration is not to 

 be found in this direction. 



Finally, to revert once more to the experiments that 

 gave positive results. It has been shown that from the 

 proximal end of a reversed femur new limb bones develop. 

 This result calls for further analysis. It is clear that 

 each level of the limb has the power to regenerate all 

 of the parts lying more distal to it, and in all proba- 



