104 C. M. CHILD 



oral lobe, with forms like figure 23 as the maximum modification 

 of form. 



Acetic acid was used only for comparison with HC1, and in two 

 concentrations m/2000 and m/5000, acting continuously from 

 the first cleavage. The forms produced were of the same type 

 and range of variation as in the same concentrations of HC1. 



With the acids, as with alcohol, it is evident that for the more 

 extreme modifications of form by differential acclimation, a cer- 

 tain degree of differential inhibition is necessary as an antece- 

 dent condition. The most extreme modifications of form by 

 differential acclimation appear in acids because differential accli- 

 mation begins earlier and progresses more rapidly than in other 

 agents, so that the relations of parts are widely altered before 

 the skeleton develops. 



GENERAL CONCLUSIONS 



The experimental data leave no possibility of doubt concern- 

 ing the effectiveness of the axial metabolic gradients as funda- 

 mental factors in the embryonic development of the sea urchin. 

 The differences in susceptibility to inhibiting agents which are 

 associated with the differences in metabolic rate at the different 

 levels of the gradients, determine on the one hand the differen- 

 tial effects of direct inhibition and, on the other, those of differ- 

 ential acclimation and differential recovery. 



Since there are no specific differences in the form-changes pro- 

 duced by the different agents used, there is no basis for the as- 

 sumption of specific action in any case. All the facts indicate 

 that the action of the various agents is essentially quantitative, 

 so far as it concerns the processes of growth and development. 

 It is very probable that the different agents do not all act in 

 exactly the same way on the sea urchin protoplasm, but the 

 point of present importance is that, however they act, whatever 

 condition or reaction complex in the system they affect primarily, 

 their general effect is a retardation or inhibition of the funda- 

 mental metabolic processes, which may be more or less com- 

 pletely reversed by. acclimation or recovery. The changes in 

 form result from the differences in effect on different regions. 



