THE RANGE OF DOMINANCE 149 



we see new individuals being localized and developing 

 where it is impossible to conceive of any internal local- 

 izing and determining factors other than quantitative 

 metabolic conditions. 



In the case of Planaria I have been able to increase 

 the frequency of biaxial heads (see Fig. 48, p. 99) in very 

 short pieces by partially narcotizing the animals before 

 cutting and keeping the pieces in a dilute solution of a 

 narcotic, e.g., chloretone, for a day or two before allow- 

 ing them to develop. Under such conditions the meta- 

 bolic rate in the pieces is of course decreased, and so 

 dominance in the direction of the original gradient is 

 still further decreased. Consequently, when the pieces 

 are returned to water and allowed to develop, the con- 

 ditions are even more favorable for the establishment of 

 the reversed gradient at the basal end, and biaxial 

 structures develop in a larger percentage of cases than 

 when the pieces are not narcotized. The effect of the 

 narcotic is simply to aid in decreasing the dominance of 

 the original apical region of the piece and so to increase 

 the probabihty of the establishment of an effective 

 reversed gradient and dominance at the basal end. 

 This experiment has not as yet been attempted with 

 Tubularia, but will no doubt be successful with proper 

 technique. 



THE EXTENSION OF DOMINANCE DURING DEVELOPMENT 



That the range of dominance undergoes extension 

 during development is evident from many facts. In the 

 young Planaria, for example, a second zooid arises at 

 the posterior end of the body when the animal is less than 

 five millimeters in length, i.e., the range of dominance 



