4IO 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



This inhibition of planarian head development by posterior section, 

 Hke inhibition of head development and destruction of zooids in Stenosto- 

 mum, represents an interference by a region posterior to the level of re- 

 constitution with a dominance already present or with the attainment of 

 the independence necessary for head formation. The decrease in length 



100 



Fig. 140. — Increase in head frequency with delay of anterior section {Dugesia doroto- 

 cephala). Ordinates, head-frequency indices (see Appendix VII); abscissae, hours; curve ab 

 from data obtained by Child with fifty pieces for each period of delay; curve cd from com- 

 bined data of students, including more than six hundred pieces (from Child and Watanabe, 

 1935a)- 



of hydranth primordium in Tuhularia with decrease in length of piece 

 evidently represents a similar, but less conspicuous and effective, inter- 

 ference. Moreover, Hyman (1916a) has shown somewhat similar relations 

 in Lumhriculus. 



In the reconstitution of a new dominant region from other body-levels 

 than the apical or anterior region there is, so to speak, a conflict between 

 the factors concerned in the reaction initiating reconstitution and those 

 tending toward maintenance of the original condition. The new domi- 



