PHYSIOLOGY: C. M. CHILD 
169 
time the axiate organic individual in its simplest terms. In fact we 
may define the axiate individual as consisting primarily in a gradient or 
gradients in rate of metabolism or of certain metabolic reactions in a 
specific protoplasm. The organic individual is not then simply a hodge- 
podge of chemical substances : it is a physico-chemical complex with one 
or more reaction gradients. If this conception of the organic individual 
is correct, then we must admit further that physiological correlation is 
primarily a matter of the transmission of chemical changes rather than 
of the transportation of chemical substances. In other words, axiate 
organic individuation or integration is of the nervous type, from the 
beginning, and the development of the nervous system is the morpho- 
logical expression of physiological conditions which were present and 
began to act at the moment when the axis first arose and which in fact 
constitute the axis in its simplest terms. Undoubtedly specific chemical 
substances, hormones, products of metabolism, or whatever we prefer 
to call them, play very important parts in organic development, but the 
individual must already exist as an orderly whole before they can act in 
any definite and orderly manner. 
We must now consider the question of the origin of the gradient, or 
more correctly of the region of high metabolic rate which determines the 
gradient. The only possible conclusion from many different lines of 
evidence is that it results from the differential action of factors external 
to the protoplasm, cell or cell mass concerned. We see gradients arising 
in nature in this way and it is possible to produce them experimentally. 
If this conclusion is correct, the so-called polarity and symmetry of 
organisms, which are in reahty morphological and physiological expres- 
sions of these gradients, are not fundamental properties of protoplasm 
and do not result from polarities or symmetries of its constituent mole- 
cules or particles. They are therefore not comparable to the physical 
polarities and symmetries which are resultants of atomic or molecular 
constitution, for they represent molar differences in metabolic condi- 
tion, which in the final analysis are of external origin. It is probable, 
however, that no cell or cell mass can continue to exist under ordinary 
natural conditions for any considerable length of time without acquiring 
at least a temporary gradient or gradients, for imiformity of action of 
external factors at all points of its surface is inconceivable, except per- 
haps for very short periods, and differences of action will result in a 
gradient or gradients. And it is certain that an orderly and definite 
sequence of events in time and space, such as occurs in organic develop- 
ment, is possible only where one or more of these metabolic gradients 
