THE ORIGIN OF THE NERVOUS SYSTEM 



or regeneration, etc., along the chief body axis. In 

 fact, evidence of one kind or another for the existence 

 of gradients of some sort as characteristic features of 

 axiate organisms appears in the literature of many 

 different and widely separated fields of investigation, 

 both physiological and morphological. This evidence 

 is perhaps the more valuable because the authors were 

 in most cases stating or describing facts rather than 

 attempting to formulate theories. In the conception 

 of the excitation gradient and its fixation in protoplasm 

 I have merely attempted to provide a general physio - 

 logical basis for the facts as they stand. 



