CONTROL OF HEAD FORMATION IN PLANARIA 5 



of Planaria it is indicated by a greatly increased susceptibility 

 of the region to KNC (Child, '13 b). This method indicates 

 that the region of the wound has been stimulated to a higher 

 rate of metabolism than any other region of the piece, but that 

 the rate is probably lower if the cut is through a lower level of 

 the gradient (Child, '14 a, '11 b). We may therefore picture 

 the first effect of section as establishing a region of very high 

 rate of metabolism immediately adjacent to the cut surfaces of 

 the piece, the rate being slightly lower in pieces from the lower 

 levels of the gradient. 



The new relations of the cells near the anterior cut surface 

 are somewhat different from the relations of the cells of the cor- 

 responding region near the posterior cut surface of the piece. 

 In the former case, since the path of nervous correlations and 

 integrative control is chiefly in the anteroposterior direction, 

 the cells near the anterior cut surface are to a considerable ex- 

 tent freed from such integrative and differentiative controls, 

 especially from those arising in regions more anterior (Child, 

 '11 c, '14 b). The cells near the posterior cut surface are, how- 

 ever, still largely under control of the piece. Attention is con- 

 fined in the present paper to the conditions controlling the cells 

 near the anterior cut surface, since they produce the new head. 



The second result of isolation to be considered is the im- 

 mediate stimulation of the remainder "of the piece. This has 

 been shown by susceptibiUty methods (Child, '14 a) and by 

 direct measurement of carbon-dioxide production (Robbins and 

 Child, '20) and oxygen consumption (Hyman, '19 b). This 

 stimulation may be considered to arise from the severing of the 

 nerve cords and is probably of nervous origin in all regions of 

 the piece. The degree of stimulation is inverse to the length of 

 the piece and inverse to its position in the gradient of the intact 

 animal; short pieces are more stimulated than long pieces and 

 pieces from posterior lower levels of the zooid are more stimulated 

 than those from the anterior higher levels. The stimulation 

 lasts for a number of hours, decreasing gradually, until within 

 a day or two it completely disappears. In an isolated piece 

 we may therefore picture the second effect of isolation as a tempo- 



