32 THE ORIGIN OF THE NERVOUS SYSTEM 



less active state and, on the other, undergoes more 

 rapid and more complete recovery from temporary 

 alteration. In short, so far as susceptibility is non- 

 specific and quantitative, it is apparently an indicator 

 of the quantitative aspects of physiological state in 

 protoplasm. Susceptibility to the higher, directly 

 lethal concentrations and intensities can be determined 

 by survival time, and in many cases loss of motor 

 activity, swelling or shrinkage of cells, or other changes 

 preceding death can be used as a check on survival time. 

 To lower concentrations, which are not directly lethal, sus- 

 ceptibility can be determined by the degree of inhibition, 

 acclimation, or recovery in growth, development, motor 

 activity, etc., in different body regions or individuals. 



The data on susceptibility have demonstrated the 

 existence in a large number of organisms of definite 

 susceptibility gradients as a characteristic, and as the 

 earliest distinguishable features of axiate pattern, not 

 only for the organism as a whole but for various axiate 

 organs and parts. In all cases the differences in sus- 

 ceptibility correspond to differences in structure, rate 

 of growth, development, and differentiation, and in cer- 

 tain cases it has been possible to show that susceptibility 

 gradients correspond to gradients in oxygen consump- 

 tion and carbon dioxide production. 



In various cases it has been possible to follow the 

 apico-basal or anteroposterior gradient from the egg 

 and to show the modifications which occur during the 

 course of development and the appearance of new 

 gradients in particular organs, or in connection with 

 agamic reproductive processes. In the simpler organ- 

 isms the susceptibility gradients are wholly or largely 



