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77/ E . 1 MKRK AX A\ 1 TURALIST 



[Vol. XLII 



bility every level has potentially the power also to re- 

 generate all the other parts of the limb proximal to 

 that level. It is difficult to show that a distal part has 

 the potentiality to produce more proximal parts, but the 

 facts make this interpretation highly probable. How 

 much of the distal end regenerates depends in part 

 on its relation to what is left in the stump, and in part 

 on the necessity of forming a distal structure. Between 

 these limits the intermediate parts are laid down. The 

 proximal cut end of a limb must have the same poten- 

 tiality of forming distal structures as has a distal end 

 and in those cases where the possibility exists of forming 

 either an anterior or a posterior structure, as in pieces 

 of lumbriculus, for example, some other relation must 

 determine that from one end of a piece a head always 

 develops, and from the other end a tail. I have sug- 

 gested that the direction of the gradations of the old 

 material (as expressed in their differentiation) is the 

 factor that regulates this result, If we apply these same 

 ideas to the special case under consideration we might 

 expect the proximal end of the leg (or any part of it) 

 to regenerate only proximal structures; in other words 

 to complete the proximal end of the femur and produce 

 a scapula at the exposed end of the leg, and, theoretically, 

 one might imagine the further development of a salaman- 

 der around the scapula as a center. The facts are the re- 

 verse. The conditions that determine in the case of the 

 reversed femur what shall regenerate of the various pos- 

 sible ones are not so simple as just described. In the 

 first place, the detachment of the femur from the rest 

 of the limb may soon lead to changes in it that cause 

 it to lose that gradation of materials on which the po- 

 larity of the new part depends. There is also the pos- 

 sibility that the polarity of the other tissues may have a 

 counterbalancing influence. But far outweighing these 

 possibilities there is another consideration of greater 

 weight. The special group of tissues found in such an 

 organ as a limb may be capable of forming only one 



