PROCESS OF REGENERATION 157 



nated, as can be done by taking very short pieces, or putting the 

 pieces in depressing agents, then the cells at the posterior end 

 can become independent, and develop a head. Biaxial heads 

 result under these conditions. On the other hand, if the old 

 piece has a high rate of metabolism, it may be able to dominate the 

 new cells growing out at the cut surfaces to such an extent that 

 neither one can produce a head but each develops a subordinate 

 part, resulting in biaxial tails. In most organisms these occur 

 in short pieces only, where the piece attains a high rate of metab- 

 olism through the stimulation from section. But in some 

 oligochaetes, particularly the earthworm, biaxial tails are pro- 

 duced by long pieces. Dynamic factors alone cannot account 

 for this result, but it is probably due to the presence of the 

 secondary gradient. 



The conception of the process of form regulation to which Child 

 has come as the result of his extensive experiments with Coe- 

 lenterates and flatworms can then be satisfactorily applied to 

 the microdrilous oligochaetes. This conception may be briefly 

 summarized as follows. The head can only arise if the cells 

 which are to form it attain physiological isolation and inde- 

 pendence from the rest of the piece. The head is a self-differenti- 

 ating system ; it does not develop in correlation with other parts 

 but is the starting point for a new system of correlations, in other 

 words, a new individual. In long pieces the head forms at the 

 anterior end of the piece because the axial gradient determines 

 that at this end alone can sufficient physiological isolation be 

 attained. In short pieces, head formation depends on certain 

 dynamic relations between the head-forming cells, and the old 

 piece, and these dynamic relations are, in turn, dependent, al- 

 though indirectly, on the axial gradient. All other parts are 

 subordinate, and arise in correlation with the head, or with an- 

 terior regions, which themselves develop in correlation with the 

 head. This statement is proven conclusively by the fact that no 

 piece ever regenerates structures anterior to its level unless a 

 head is formed first at its anterior end. The response of cells or 

 groups of cells to isolation, physical or physiological, is the 

 production of an apical region or head; this response is the 



