40 THE ORIGIN OF THE NERVOUS SYSTEM 



both organ and agent, but in the simpler forms and the 

 earlier stages the similarity of the susceptibility gradients 

 in widely different organisms and with a great variety 

 of agents renders their nonspecific character sufficiently 

 clear, and many lines of evidence, both direct and 

 indirect, indicate their relation to rate of metabolism or 

 oxidation. The nature and degree of that relation in 

 each particular case and for each particular agent 

 remains of course to be determined by other methods of 

 investigation. The susceptibility method has served to 

 bring to light certain characteristic features of organismic 

 pattern which have not previously been clearly recog- 

 nized, viz., the gradients, but conclusions concerning 

 the exact nature of these gradients are possible only on 

 the basis of all the different lines of evidence obtainable, 

 and at present of course cannot be final. 



THE EVIDENCE FROM RATE OF PENETRATION 



In many forms, both plants and animals, in which 

 susceptibility gradients exist, corresponding gradients in 

 the rate of penetration of certain substances, particu- 

 larly the vital dyes neutral red and methylene blue, 

 which have been most extensively used in these experi- 

 ments, have also been demonstrated. 1 The existence of 

 these gradients in rate of penetration raises the question 

 whether the susceptibility gradients are not primarily 

 gradients in permeability of the protoplasmic surfaces 

 to the agents used. While there is no doubt that a 

 gradient in permeability is one aspect of the axial 

 gradient, our conception of the relation between per- 



1 Much of this work has been done by Mr. J. W. MacArthur and 

 is not yet published, For some observations on algae see Child (1919c). 



