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 
