DIFFERENTIATION OF NERVOUS STRUCTURE 115 



tion modifies the structural substratum, and this in 

 turn modifies further function and so on. If the effect 

 of functional activity on the structural substratum is 

 ever completely reversible it is only or chiefly in the 

 fully developed organism where a dynamic equilibrium 

 between structure and function is attained or approached. 

 Development represents the progress from a disturbed 

 protoplasmic or cellular equilibrium to an organismic 

 equilibrium, or, as in physiologically isolated regions or 

 physically isolated pieces, the progress from a disturbed 

 organismic equilibrium to a new equilibrium, and the 

 metabolic or physiological gradients constitute the 

 framework or ground plan on which this progress takes 

 place. 



The primary germ layers represent the primary 

 regional differentiation with respect to the gradient 

 pattern. They result from the differences in rate of 

 metabolism at different levels of the gradient and con- 

 stitute the basis for a chemical or transportative relation 

 which arises sooner or later, when ectoderm grows at the 

 expense of the yolk in the entoderm, or of food digested 

 by the entoderm. This transportative relation between 

 ectoderm and entoderm is, however, secondary and the 

 consequence of the differences in rate of activity between 

 them. The nervous system, on the other hand, repre- 

 sents the primary differentiation with respect to the 

 functions of excitation and transmission of the gradients, 

 and as the gradients represent the primary integrating 

 factor in the development of the organism, the nervous 

 system as the differentiated mechanism of this primary 

 integrating function necessarily becomes the chief organ 

 of integration. 



