RECONSTITUTIONAL PATTERNS IN EXPERIMENT 



409 



extremely short pieces of some species posterior section has httle or no 

 effect; in others there is inhibition, but less than in D. dorotocephala. 

 The species differences probably result chiefly from differences in rate or 

 intensity of anterior activation following section, in relation to the effec- 

 tiveness of the inhibiting factor. 



Fig. 139, A, B. — Effect of delay of posterior section on planarian head frequency {Dugesia 

 dorotocephala). A, outline indicating levels of section; B, graph of increase in head frequency 

 with delay of posterior section; ordinates, head-frequency indices (see Appendi.x VII), abscissae, 

 hours of delay; curves ah and cd, twenty-five pieces for each period of delay, from data obtained 

 by Child with different stocks; ef, from combined data obtained by students in laboratory 

 class work, more than seven hundred pieces. Irregularities are chiefly due to differences in 

 length of pieces (from Child and Watanabe, 1935a). 



Head regeneration in the Dendrocoelidae and some other forms does 

 not occur posterior to a certain body-level, irrespective of posterior sec- 

 tion and length of piece, probably because cells of more posterior gradient- 

 levels do not react sufficiently to isolation by section to develop a head. 

 They are capable, however, of giving rise to posterior ends. 



