CONTROL OF HEAD FORMATION IN PLANARIA 3 



such as KNC, is a fundamental factor in determining head fre- 

 quency (Child, '11 a, '19 a, '20 a). 



Bj^ investigation of the regeneration of pieces of the same length 

 from different regions of the body of animals of the same size, 

 Child showed that there is a definite gradation in head frequency 

 in such pieces, decreasing from the head posteriorly to the region 

 at which fission usually occurs, indicating there the posterior 

 limit of the first zooid. Then it rises abruptly to decrease again 

 toward the end of the second zooid (Child, '11 a, '11 b). 



These two facts, that conditions controUing regeneration are 

 physiological and quantitative and that there is an anteropos- 

 terior gradation in head frequency in each zooid, when jointly 

 considered lead to the inference that there must be some sort 

 of quantitative physiological gradient along the axis of the 

 animal. By means of certain poisons in solutions strong enough 

 to kill slowly. Child ('11 b) was able to demonstrate a well- 

 marked anteroposterior gradient in survival time. The reagent 

 most extensively used for this purpose was KNC, which is known 

 to interfere with the oxidative reactions. In appropriate solu- 

 tions of this reagent, as well as others, the anterior end of the 

 animal dies first, indicating that there the rate of oxidative reac- 

 tions is most rapid, since the protoplasm is most susceptible 

 there. 1 The death process then progresses posteriorly in each 

 zooid, indicating the decreasing rate of metaboUsm from ante- 

 rior to posterior (Child, '11 b, '13 a, '13 b). 



Child's first conclusion, based on his results on regeneration 

 and this differential susceptibility to poisons, that the gradient 

 made evident by the data is a rough index of the relative rate 



1 The differential susceptibility to external agents of regions of different rates 

 of metabolism has been itself the subject of extended investigation and discussion 

 (Child, '13 a '14 b, '15 a, '20 b, and other papers; Bellamy, '19; Hyman, '19 a, 

 '21 a). The findings have been that regions of high rate of metabolism are more 

 affected by certain ranges of concentration or intensity of action of external 

 agents than regions of lower rate. To certain ranges of lower concentrations or 

 lower intensities of action, however, the regions of higher rate of metabolism are 

 better able to acclimate or recover from the effects of the agent or condition and 

 consequently show eventually less effect of such treatment than regions of 

 lower rate. 



