106 C. M. CHILD 



ticular case, or, more correctly speaking, a certain limited range 

 of variation in metabolic relations, and it is normal, merely be- 

 cause, under the usual conditions, this range of variation is not 

 exceeded. 



Not only differential growth, but local differentiation, may 

 occur at different points of a metabolic gradient. Some of the 

 evidence in support of the view, that local differentiation re- 

 sults, in the final analysis, from the differences in metabolic 

 condition which arise at different points of a gradient, in conse- 

 quence of the differences in rate of reaction, has been considered 

 elsewhere (Child, '15 c, pp. 127-169, 183-188). If this view be 

 correct, 'normal' localization of differences in development is 

 like general form and proportion merely, one particular case or a 

 certain limited range of cases representing a certain limited range, 

 the ' normal' range of variation in the essential metabolic 

 conditions. 



The experimental methods used in the present paper serve 

 merely to alter the differences in metabolic rate between differ- 

 ent parts and so to alter the resulting balance and therefore the 

 spatial order as expressed in growth, form, and differentiation. 

 In differential inhibition the slope of the metabolic gradient is 

 decreased, the gradient is more or less completely leveled down, 

 because the regions of high rate of reaction are more susceptible 

 to the action of the inhibiting agent and so undergo a greater 

 decrease in rate, than the regions of lower rate. In consequence 

 of this decrease in slope of the metabolic gradient, the differ- 

 ences in metabolic rate between different points of the gradient, 

 and therefore the differences in rate and amount of growth, 

 become less than in the normal animal, and the relative size and 

 proportions of parts along any axis are altered, those parts which 

 represent regions of high rate of reaction becoming relatively 

 smaller and those which represent regions of low rate, relatively 

 larger. As this decrease in slope of the metabolic gradient 

 progresses the gradient becomes less effective as a factor in dif- 

 ferentiation, and local morphological features along its course 

 may become less and less marked and finally disappear, or more 

 correctly, fail to appear. 



