154 LIBBIE H. HYMAN 



axial gradient also which determines that the head shall arise at 

 the anterior end of the piece (Child, '14 e, p. 73). 



The objection may be raised to these statements that in some 

 of the oligochaetes which I have been considering, the axial 

 gradient is such as to be higher at the posterior end than at the 

 anterior end of long posterior pieces. Now there appears to be 

 a correlation between the extent of the characteristic posterior 

 rise in metabolic rate, and the capacity for head formation. Thus 

 in Dero, where the posterior rise is of small extent, head formation 

 occurs at all levels; in Lumbriculus, where the region of high rate 

 occupies more than the posterior half of the body, head formation 

 tends to be inhibited in this region ; and in the tubificids where 

 nearly all of the body is involved in the ascending gradient, head 

 formation is impossible except at extreme anterior levels. In 

 my opinion, however, the ascending gradient plays only a minor 

 role in morphogenesis. I wish to point out that a similar series 

 with regard to head formation can be found in the Turbellaria; 

 Planaria maculata regenerates normal heads at all levels in reason- 

 ably short pieces; P. dorotocephala in pieces of similar size forms 

 normal heads at anterior levels only; Dendrocoelum lacteum 

 will not produce heads at all behind the anterior third of the 

 body; while in many polyclads head formation ceases posterior 

 to the cephalic ganglia. In these forms, there is no posterior 

 region of high rate to account for the facts, but the explanation 

 lies in all probabilitj^ in the character of the primary gradient, 

 and in the rate at which new tissue grows out. The posterior 

 rise in rate in oligochaetes is the expression of the increasing 

 youth of cells in the posterior direction; it must be regarded as a 

 secondary gradient superposed on the primary integrative gradi- 

 ent in the nervous system. Evidence for this point of view is 

 found in the experiments on stimulation after section, where it 

 was shown that these posterior regions of high rate are affected 

 in the same way although not to the same extent as regions of 

 low metabolic rate, and are, therefore, like the latter, subordinate 

 parts dependent on correlation with more anterior regions. The 

 fact, however, that they are stimulated to a less degree than the 

 regions of low rate indicates that they possess a certain slight 



