PHYSIOLOGICAL DOMINANCE 117 



ing tip is, I believe, essentially the same process as the 

 formation of the new head in Planaria. In cases where 

 wound callus develops, new growing tips may arise in 

 that. In the formation of a new growing tip in callus 

 tissue (pp. 85-86) and its later connection with other 

 parts of the plant we have again a process very similar 

 to the formation of a head in a piece of Planaria, and the 

 development under its dominance of other parts, so far 

 as they are not already present. In both cases the new 

 apical region is not determined by other parts but 

 develops independently of them, and its later relations 

 to them are determined by its own dominance. 



SOME MODIFYING AND LIMITING FACTORS IN ANIMAL 

 RECONSTITUTION 



The development of double or biaxial apical regions 

 from short pieces has been discussed above (pp. 98, 99). 

 In some cases biaxial basal regions 

 instead of apical regions arise from 

 pieces. Pieces of the stems of certain 

 hydroids sometimes produce stolons 

 at both ends, biaxial tails have been FlG S9 ._Experi- 



observed in short pieces of Planaria mentally determined 



by Morgan, and I have been able to "constitution of 



.. . biaxial basal ends in 



produce them experimentally in some piece of Planafia . 

 cases (Fig. 59) by altering the relations 

 of metabolic rate between the regions x and y (Fig. 58) 

 with the aid of narcotics. In the earthworm and related 

 forms various investigators have observed the develop- 

 ment of tails at both ends of pieces from the more basal 

 regions of the body. My own experiments indicate that 

 when the development of the new tissue at a cut end of 



