162 C. M. CHILD. 



rate of metabolism or oxidation mean no more than that rate of 

 metabolism or oxidation is so far as the evidence goes a factor 

 in the conditions which determine susceptibility, but it is not 

 claimed that this conclusion is universally valid. Undoubtedly 

 in the more highly differential organisms and in more advanced 

 stages of development the qualitative differences in different 

 organs may determine differences in susceptibility which are 

 more or less specific as regards both organ and agent, but in the 

 simpler forms and the earlier stages the similarity of the suscep- 

 tibility gradients in widely different organisms and with a great 

 variety of agents renders their non-specific character sufficiently 

 clear, and many lines of evidence, both direct and indirect, indi- 

 cate 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 recognized, 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. 



Susceptibility in Relation to Permeability. In many forms, 

 both plants and animals, in which susceptibility gradients exist, 

 corresponding gradients in the rate of penetration of certain sub- 

 stances, particularly 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 perme- 

 ability 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- 

 meability and the gradient must depend very largely upon the 

 terms in which we define permeability. If permeability is de- 

 pendent only on the physical condition of the limiting surface 



i Much of this work has been done by Mr. J. W. MacArthur and is not yet 

 published. For some observations on algae see Child, '200. 



