$6 THE ORIGIN OF THE NERVOUS SYSTEM 



embryonic relations have undergone more or less alter- 

 ation, even in Planar ia. 



With the appearance of more or less specific relations 

 between particular regions or organs and particular 

 agents, the value of the susceptibility method as a 

 means of distinguishing general quantitative differences 

 and relations is of course greatly decreased. In the 

 higher animals apparent specificity of relation between 

 particular organs and particular agents is much more 

 evident than in lower forms and becomes increasingly 

 complex with the progress of differentiation, but even 

 in these forms the general nonspecific susceptibility 

 relations appear in the earlier developmental stages, at 

 least in all forms examined. 



While caution is always necessary in interpreting 

 the data of susceptibility in nonspecific quantitative 

 terms it has become more and more evident as the data 

 have accumulated that general nonspecific susceptibility 

 relations do exist, particularly in the simpler organisms 

 and in the earlier stages of development, and that they 

 are indications of fundamental physiological features of 

 organismic pattern. 



It has also been possible to control and modify 

 development in definite predictable ways through the 

 differential susceptibility of different levels of the axial 

 gradients. Such modifications consist in differential 

 inhibition, differential acceleration, differential acclima- 

 tion and differential recovery, each representing a 

 definite teratological type. 



In cases of differential inhibition with respect to a 

 gradient the degree of inhibition varies directly with 

 the susceptibility and activity of different levels. The 



