PHYSIOLOGICAL DOMINANCE iii 



are stimulated ; consequently the more basal the level of 

 a piece the more its rate is increased by section. I 

 have also found that the shorter a piece, the higher the 

 metabolic rate after section. Long pieces are stimulated 

 but little, except at the ends, chiefly the apical, but in 

 short pieces the rate increases greatly. 



Another simple experiment^ shows that under ordi- 

 nary conditions it is determined within three to six 

 hours after section whether or not a head will develop 

 on a piece. This is during the period of stimulation of 

 the piece, and when we compare head-frequencies and 

 metabolic rates during the period of stimulation follow- 

 ing section, we see that the higher the metabolic rate 

 in the piece as a whole, i.e., the region y, Fig. 58, the less 

 likely a head is to develop, and vice versa. The head- 

 frequency is lower in more basal pieces such as c than 

 in more apical pieces like a, and in shorter than in longer 

 pieces, because the metabolic rate in the region y is higher 

 at the time of determination of the course of develop- 

 ment. Pieces from young or starved animals also have 

 a higher metabolic rate'' and a lower head-frequency 

 than similar pieces from old or well-fed animals. 



These facts may seem to involve a paradox, but 

 their interpretation is actually simple. The two regions 

 X and y (Fig. 58) of the piece behave differently after 

 section. The cells of x are so extremely affected by 

 the presence of the wound and the altered conditions 

 that they rapidly dedifferentiate and begin to divide 

 and grow, and so approach or attain an embryonic 



^ Child, "Studies on the Dynamics of Morphogenesis. VIII," 

 Jour, oj Exp. Zool., XVII, 1914. 



' Child, Senescence and Rejuvenescence, 19 15, pp. 155-63. 



