62 



C. M. CHILD 



Short pieces from the middle region of the body, i.e., the pos- 

 terior region of the first zooid of Planaria dorotocephala, such for 

 example, as ab, be and cd in figure 1, give rise in 98 to 100 per cent 

 of the cases, under ordinary conditions, to headless forms (Child 



'11 b, '11 c), i.e., no outgrowth of new tissue occurs at the anterior 

 and beyond the healing of the wound. 



On the other hand, long pieces with anterior ends at the same 

 levels as those of the short pieces (pieces like ae, af, ce, cf) pro- 

 duce under the same external conditions 98 to 100 per cent of 

 animals with fully developed normal heads. 



