REGENERATION 7 



if it be stuck into the sand in the reverse position a head arises at 

 the end which is uppermost, where the roots arose previously, and the 

 previous head-end now sends out roots. By suspending a beheaded 

 stalk horizontally in the water a head can be caused to develop at 

 each end of the stalk, so that we must assume that every part of the 

 polyp is, under some circumstances, capable of developing a head, and 

 that it must be ' circumstances ' — in this case gravity, contact with 

 earth or with water, and the mutual influence of the parts of the 

 animal upon each other — which decide what is to be produced. 

 Loeb, who was the first to observe this form of regeneration, called it 

 heteromorphosis, to express the fact that particular parts of the 

 animal might be produced at quite different places from those 

 originally intended for them. 



It would certainly be erroneous to range these cases of hetero- 

 morphosis against the determinant theory, but they certainly do not 

 aflbrd any special evidence of its validity as an interpretation, for all 

 that we can say here again is that all, or at least many, cells of the 

 animal must contain the full determinant-complex of the ectoderm, 

 and others those of the endoderm, and that particular groups of 

 determinants become active when they are affected by certain external 

 or internal liberating stimuli. In regard to such animals the theory 

 is hardly more convincing than the rival theory, that the faculty of 

 regeneration is a general property of living substance, which does not 

 attain to equally full expression everywhere, because it is met by ever- 

 increasing difficulties involved in the increasing complexity of struc- 

 ture. The validity of the theory only begins to be seen when we 

 deal with cases where it is demonstrable that every part cannot bring 

 forth every other, where the power of regeneration is limited, and 

 occurs only in definite parts in a definite degree, and can only start 

 from particular parts. Here the assumption of a general and primary 

 regenerative capacity fails. Any one who insists, as 0. Hertwig does, 

 that the idioplasm in all cells of the body is the same, can always 

 plead that, in the cases in which regeneration does not occur, the 

 fault lies, not in the regenerative capacity, but in the absence of the 

 adequate liberating stimuli, and at first sight it does seem as if this 

 position were unassailable. We shall find, however, that there are 

 facts which make Hertwig's interpretation quite untenable. 



My own view is that the regenerative capacity is not something 

 primary, but rather an adaptation to the organism's susceptibility 

 to injury, that is, a power which occurs in organisms in varying 

 degrees, proportionate to the degree and frequency of their liability 

 to injury. Regeneration prevents the injured animal from perishing, 



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