70 C. M. CHILD 



500 cc. or 1 1. corked Erlenmeyer flasks filled almost to the 

 cork, and solutions were renewed every twelve hours where the 

 period of action was longer than that. It has been previously 

 determined that development would proceed normally in such 

 corked flasks in well aerated water, renewed every twelve hours, 

 provided the number of eggs or embryos was not too great, 

 and particular care was taken in the experiments that the num- 

 ber should not be excessive. In the experiments involving re- 

 covery after temporary action of the reagent, lots of eggs or 

 embryos were returned to sea water at certain intervals, with 

 four or five changes to remove as far as possible all traces of the 

 reagent. 



THE EXPERIMENTAL MODIFICATIONS OF DEVELOPMENT 



If the above statements concerning the relations between di- 

 rect susceptibility, acclimation and recovery and metabolic rate, 

 are correct, and if metabolic gradients are actual and effective 

 factors in embryonic development, it is evident that three possi- 

 bilities exist for the modification of development by the action 

 of inhibiting agents. These possibilities are: first, direct dif- 

 ferential inhibition of development by relatively high concen- 

 trations or highly toxic agents, in which case the degree of in- 

 hibition should vary directly with metabolic rate; second, in- 

 direct differential inhibition through differential acclimation to 

 lower concentrations or less toxic agents, in which case the degree 

 of inhibition should vary inversely as the metabolic rate; third, 

 indirect differential inhibition through differential recovery after 

 temporary action of the agent. 



The experimental results realize expectation. In the forms 

 produced by direct differential inhibition, the development of 

 apical, anterior and median regions is more inhibited than that 

 of basal, posterior and lateral regions, while, in the forms result- 

 ing from differential acclimation, the development of basal, 

 posterior and lateral regions is more inhibited than that of apical, 

 anterior and median. The forms resulting from differential 

 recovery are, in general, similar to those resulting from differential 

 acclimation. 



