426 S. J. Holmes 



region of A will tend to mold it in the direction of the missing 

 parts. In so far as B, G and F are modified through the loss of 

 the missing part, their influence on the tissue in the region of A 

 will come to be modified, and they will, in turn, modify the cells 

 lying next to them. But, as there is a tendency for the modifica- 

 tion produced by the loss of A , to spread successively to other 

 parts, there is also a tendency, according to my theory, toward 

 the checking and reversal of this process. If the loss of A tends 

 to modify B, F and G, the presence of F, C and D tends to hold 

 them in place, and in so far as these are maintained through this 

 influence they tend to mold the tissue in the position of A into 

 the form of the missing part; and m so far as this is so molded 

 its modifying influence on B, F and G is diminished. How the 

 process works out depends naturally on the degree of specification 

 of the parts, whether or not new tissue is formed in the place of 

 the missing part, and perhaps other factors. If the organism is 

 plastic and its parts have not acquired an irretrievable set which 

 prevents further modification, it may be entirely worked over 

 in consequence of the disturbance of its social pressure in the 

 vicinity of the missing part, thus leading to redifi^erentiation, or 

 morphallaxis. Whether we have morphallaxis or regeneration 

 in a narrower sense may depend, among other things, upon the 

 degree of specification of the parts. As I have suggested in my for- 

 mer paper (p. 288), and as Child has maintained more at length, 

 regeneration as opposed to "redifferentiation, increases as func- 

 tional specification of the tissues increases or, in other words, 

 the greater the degree of diff"erentiation — the visible result of 

 functional specification — the less likely is extensive functional 

 substitution and consequent redifFerentiation. " 



This, it seems to me, is very much what one might expect ac- 

 cording to the theory of regeneration I have outlined. Replace- 

 ment of ^, according to Child, "can occur only when the relation 

 is largely one-sided, i.e., when A is dependent on B-F , but these 

 latter are not to any marked degree dependent on A . In this case, 

 and in this case only, will the social pressure force the undiff"er- 

 entiated cell to differentiate into something like A." Where 

 rediflPerentiation from new tissue is concerned, as in the present 



