LARVAL DEVELOPMENT IN THE SEA URCHIN 113 



indicates very clearly that differential susceptibility along meta- 

 bolic gradients plays a very important part in their production. 

 And as I have shown for the sea urchin, the same agent or con- 

 dition may not only produce different degrees of modification in 

 a particular direction, according to concentration, period of ac- 

 tion, etc., but may produce modifications in opposite directions, 

 the .one direction representing differential inhibition, the other 

 differential acclimation or differential recovery. On the other 

 hand, very different agents and conditions may produce similar 

 modifications of form, because the general effect upon metabo- 

 lism is not specific, but quantitative. The facts indicate that 

 the fundamental factors concerned in these modifications are 

 alterations in one way or another of the rates of reaction and so 

 of the metabolic relations of parts. 



As a matter of fact, experimental teratogeny affords the most 

 conclusive evidence of any field of investigation for the funda- 

 mental significance of metabolic gradients in orderly develop- 

 ment and differentiation, for the modification and control of 

 development by modification and control of the metabolic rates 

 and relations in these gradients enables us to determine and test 

 their effectiveness. Experimental teratogeny has suffered from 

 the failure to find a general foundation for experimental pro- 

 cedure, analysis and interpretation, but from the view point 

 which we attain with the aid of the conceptions of metabolic 

 gradients and differential susceptibility, a wide field lies before 

 us ready for logical experimentation, analysis and synthesis. 



SUMMARY 



1. If the directions or axes to which the order apparent in 

 development is related are fundamentally gradients in rate of 

 metabolic reaction, it must be possible to alter the developmental 

 order as expressed in form, proportion and localization of parts 

 by altering the relations of rate of reaction along these axes or 

 gradients. 



2. The general relation between susceptibility to a great va- 

 riety of agents and conditions which retard or inhibit metabolic 

 reaction in one way or another and rate of metabolic reaction, 



JOURNAL OF MORPHOLOGY, VOL. 28, NO. 1 



