n8 INDIVIDUALITY IN ORGANISMS 



a piece is completely dominated by the piece it gives 

 rise to a basal structure. Such dominance means simply 

 that the old tissue has a high enough metabolic rate to 

 determine the direction of the gradient in the new tissue. 

 In Planaria the development of tails at both ends of a 

 short piece is apparently due simply to the fact that the 

 metabolic rate in the piece is high enough so that the 

 new tissue does not become dominant at either end but 

 develops under the control of the old tissue. Dr. Hyman 

 has found that the conditions determining the formation 

 of double tails in Lumbriculus seem to be essentially the 

 same as in Planaria, though the factors which produce 

 them are somewhat different. She has been able to 

 determine experimentally to some extent the produc- 

 tion of heads instead of tails in such pieces by methods 

 similar to those which I have employed for altering 

 head-frequency in Planaria. She has also observed the 

 development of structures intermediate between head 

 and tail, or rather inhibited, rudimentary cephalic ends, 

 in which certain caudal characteristics appear later. 

 These are apparently cases in which the new tissue was 

 at first to some extent independent but later became 

 subordinated to the old. 1 



The absence of any outgrowth at the apical end of a 

 piece, as in the headless forms of Planaria (Figs. 52, 53), 

 occurs when head-formation is completely inhibited, 

 but the degree of dominance is not sufficient to deter- 

 mine development as a tail. In such cases local con- 

 ditions at the cut apparently determine the result and 

 the wound simply heals. In some other forms the wound 



1 I am indebted to Dr. Hyman for permission to use these unpub- 

 lished data. 



