NERVOUS SYSTEM AND OTHER PARTS 127 



physiological situation and both are consequences of the 

 fact that this portion of the central nervous system 

 develops from the high end of the primary gradient, 

 i.e., from the region of the body in which activity is 

 most intense during the earlier stages. 



The experiments of Waelsch (1914) and Fischel"(i9i4) 

 on birds and Amphibia indicate that in these forms also 

 the earlier embryonic stages of the central nervous system 

 arise independently of other parts. Peripheral parts 

 undoubtedly do exercise some determining influence on 

 the later differentiations in the central nervous system of 

 vertebrates, as is shown by the experimental work 

 of Shorey (1909) on birds and Amphibia and numerous 

 data on the retardation or inhibition of development of 

 central organs in the absence of peripheral function in 

 man and mammals. This, however, is unquestionably 

 a secondary condition and associated with the decrease 

 in "steepness'' or decrement in the chief axial gradients 

 with the advance of development. In earlier stages, 

 and in many of the lower organisms throughout life, 

 the difference in irritability or metabolic condition is so 

 great that transmission is chiefly or wholly in one direc- 

 tion, i.e., down the primary or chief gradients because 

 most excitations cannot run very far up the gradients 

 without disappearing as excitations in the higher 

 metabolic level (see pp. 97-99). With the ontogenetic 

 and phylogenetic increase in conductivity along the 

 gradients and the consequent decrease in decrement, 

 excitation may be transmitted to greater and greater 

 distance up a gradient, and peripheral parts or lower 

 levels of a gradient become increasingly important 

 factors in determining differentiation and physiological 



