REGENERATION. 207 



To repeat ; when a piece is cut out of the body of hydra a 

 molecular change takes place in the protoplasm of such a sort 

 that the entire mass is changed over into a structure that rep- 

 resents in its structural basis a new hydra. It is this molecu- 

 lar change that, dominating the subsequent development, seems 

 to control it, and gives us the impression of formative processes 

 at work. On my view, the formative processes are only the 

 expression of the physical, molecular structure that has been 

 assumed by the piece. 



It will be seen at once how the same conception may be 

 applied to those cases of regeneration in which a knob of new 

 tissue appears and the missing part, or a portion of it, is regen- 

 erated. After the new cells have been formed they will have 

 the same relations to each other and to the old part that the 

 cells have in the isolated piece of hydra. The new part does 

 not form a new whole, but only a part, simply because it is con- 

 nected with the old part by the same kind of molecular union 

 as are the cells of the new part to each other. Whether all the 

 missing parts, as in the limb of the newt, or whether only a 

 part, as in the head of the earthworm, will develop out of the 

 new material, will be determined by the volume of the new 

 material that forms, and its relation to the structural pecul- 

 iarity of the new part. This sounds vague, perhaps mystical, 

 but I think it can be given a real meaning. If, for instance, 

 five segments are cut off from the anterior end of the earth- 

 worm, the new material that forms suffices to make five seer- 

 ments, but if ten be cut off, the new material is still only 

 sufficient, owing to some molecular peculiarity, to make five.^ 

 In the limb of a newt we must suppose that at every level 

 enough new material is formed at first to make possible the 

 formation of any part of the limb. 



This analysis of one of the problems of regeneration has 

 been undertaken in order to see if it is possible at the present 

 time to construct an hypothesis that can bring under one point 

 of view many isolated observations. It is offered as a working 



1 There is a difficulty, perhaps, in accounting for only two segments coming 

 back when only two are cut off, and not five, but if it can be shown that less new 

 material is formed the difficulty is avoided. 



