2 LIBBIE H. HYMAN AND ALBERT E. GALIGHER 



bility method is also used to control morphogenesis during 

 regeneration or development. For this purpose less concentrated 

 solutions are employed which do not kill, but depress. This 

 depression is differential; consequently, when regenerating pieces 

 or eggs or embryos are exposed to the proper concentrations, 

 morphological modifications result which are predictable and 

 explicable on the basis of the susceptibility gradient. That dif- 

 ferential susceptibility to toxic solutions is correlated with dif- 

 ferences in metabolic rate is shown by the fact that conditions 

 which are known to modify metabolic rate also modify suscep- 

 tibility and in the same direction. 



3. Indirect susceptibility inethod. The death gradient of organ- 

 isms in dilute concentrations of toxic substances such that 

 death occurs only after several days' exposure is exactly the 

 opposite of the death gradient in quickly lethal solutions. This 

 shows that the susceptibility results are not due to differential 

 permeability, since in that case it would be impossible to ac- 

 count for the difference in the action of solutions of different 

 concentration. The reversal of the gradients in dilute solu- 

 tions is believed to be due to differential acclimation. Differ- 

 ential acclimation also accounts for certain modifications 

 induced during regeneration and development exposure to the 

 proper concentrations of toxic substances. 



4. Indophenol method. When organisms are exposed to a 

 mixture of dimethyl-7?-phenylene-diamine and a-naphthol a 

 blue precipitate of indophenol is produced. It is generally 

 accepted that the formation of this precipitate depends upon 

 the presence of oxidative processes. The depth of color 

 produced along the axes of organisms through this reaction 

 exhibits the same gradation as evidenced by the direct suscepti- 

 bility and other methods. This method has been used only 

 for embryonic stages and small forms. 



5. Potassium permanganate method. When exposed to solu- 

 tions of potassium permanganate organisms reduce it to man- 

 ganese dioxide. The capacity to carry out this reduction exhibits 

 the same gradation along the axis as appears by other methods. 



