66 C. M. CHILD 



urchin. It shows how, and to what extent, it is possible to 

 control and modify development by means of the differential 

 action of external factors on different regions of these gradients. 



THE NATURE AND PURPOSE OF THE EXPERIMENTS 



The existence of an axial metabolic gradient, at least in the 

 major or polar axis with the apical region as the region of high- 

 est rate of reaction, or, perhaps, as possessing the capacity for 

 the highest rate of reaction has been demonstrated in many 

 species of animals and plants (Child, '13 b, '14 a, '15 a, '15 c, 

 Chap. Ill, '16 a, '16 c; Hyman, '16), including the develop- 

 mental stages of the sea urchin. This demonstration has been 

 made possible by the differences in susceptibility to various 

 agents which are correlated with the differences in metabolic 

 rate and protoplasmic condition at different levels of the gradient. 



So far as this relation between susceptibility to inhibiting 

 agents and metabolic rate has been investigated, it is as follows. 

 In concentrations of cyanides, narcotics so far as tested, acids, 

 and, under certain conditions, alkalis, which kill, not immediately 

 but rapidly enough so that the organism does not become accli- 

 mated or acquire a tolerance to them, the susceptibility varies 

 directly with the general metabolic rate or with the rate of cer- 

 tain fundamental metabolic reactions. To low concentrations, to 

 which more or less acclimation is possible, the degree and rapidity 

 of acclimation vary in general directly with the metabolic rate — 

 with certain exceptions (Child, '14 b) — and within certain limits 

 of low concentration the parts least acclimated die, while those 

 more fully acclimated remain alive, so that in the long run the 

 susceptibility varies inversely as the metabolic rate (Child, '13 a, 

 '15 b, Chap. III). 



In Planaria, for example, it has been shown (Child, '13 b) that 

 to the higher concentrations of cyanides and narcotics the sus- 

 ceptibility decreases from the anterior end posteriorly, while 

 in the very low concentrations the degree of acclimation is 

 greatest at the anterior end (Child, '12), and in the long run 

 susceptibility is greatest at the posterior end and decreases 

 anteriorly. 



