64 THE ORIGIN OF THE NERVOUS SYSTEM 



one region serves to some extent to excite adjoining 

 regions, that transmission from the region of increased 

 activity will occur. In the absence of highly specialized 

 conducting paths, we find that protoplasmic excitation 

 apparently undergoes a decrement in intensity or 

 effectiveness with increasing distance from the point of 

 origin, so that an excitation gradient results. Many 

 facts, some of which are considered in the following 

 chapter, indicate that the physiological relation resulting 

 from the differential exposure of a cell or cell mass is 

 primarily a relation of excitation and transmission and 

 that the resulting gradient is essentially the fixation or 

 establishment of an excitation-transmission gradient in 

 the protoplasm through the modification of the proto- 

 plasmic subtratum by the persistence or repetition of 

 the differential exposure. From this viewpoint the 

 gradient represents the most primitive sort of excitation- 

 transmission relation and its effects upon protoplasm. 



CONCLUSION 



It is perhaps necessary to point out that the con- 

 ception of axiate pattern as primarily a quantitative 

 gradation in physiological condition is nothing more 

 than an .attempt to interpret certain aspects of the 

 physiology of development. The axial gradients do 

 not create anything, they are not the " cause" of growth 

 or differentiation in the organism, they do not determine 

 what organs shall develop in a particular protoplasm. 

 Granting their existence, and granting the significance 

 which I have assigned to them, the gradients represent 

 merely certain physiological conditions, under which 

 the hereditary mechanism of a protoplasm gives rise 



