400 PATTERNS AND PROBLEMS OF DEVELOPMENT 



the whole of zooid 1.2. with head in early developmental stage, anterior 

 to zooid 2.7, with more advanced head. Although the anterior end of the 

 posterior part of zooid i.i. is capable, under other conditions, of head 

 development, it is completely inhibited, and a gradual reduction and sepa- 



A B C 



Fig. 135, y4-C.— Planarian head regeneration at different levels, following decapitation 

 and anterior partial longitudinal splitting; explanation in text. 



ration into cells, first of the part of zooid i.i., then of zooid 1.2., results 

 (Fig. 136, B-E). The cells from these zooids accumulate in the pseudocoel 

 and gut of the dominant zooid 2.1. and in zooid 2.2. posterior to it and 

 gradually disappear, apparently serving as nutritive material, for these 

 zooids grow rapidly. That the head region alone of an older zooid can 

 bring about this destruction of headless and younger complete zooids 



