PHYSIOLOGICAL BASIS OF PATTERN 25 
dients, not because it is assumed that they are purely 
or primarily metabolic in character, but because our 
knowledge of protoplasm in general indicates that 
metabolism, and more specifically oxidation, is a funda- 
mental factor in life and the chief source of the energy 
of living organisms and because the data of observation 
and experiment indicate that differences in the rate of 
metabolism and particularly of oxidation are character- 
istic features of these gradients. Protoplasm is a 
system in which the chemical reactions of metabolism 
are so intimately associated with other factors, e.g., 
colloid dispersion, active mass of enzymes, permeability 
of limiting surfaces, electrolyte, and water content, 
etc., that to attempt to distinguish one particular factor 
rather than another as primary is at present impossible. 
The axial gradients are not simply oxidative or meta- 
bolic gradients but gradients in the general physiological 
state of the particular protoplasm concerned, and in 
this physiological state oxidation and associated with it 
other metabolic reactions are important factors. The 
term "metabolic gradient" as used in this connection 
means only that the metabolic factor is a characteristic 
feature of the gradients. The term "physiological 
gradient," which avoids all specific implications, might 
be substituted for it. 
In the development and differentiation of the axiate 
pattern the apical region or head of the organism arises 
from the region of greatest activity or highest metabolic 
or oxidative rate, the "high end" of the major or polar 
gradient, and other organs at different levels of the 
gradient. The simplest forms of radial symmetry in 
axiate organisms in which there is no differentiation 
