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 
