PHYSIOLOGICAL GRADIENTS. l8l 



most general terms than with such specialized processes. Exci- 

 tation in the broad sense is apparently an increase in the rate of 

 living or at least of that aspect of life which concerns energy 

 liberation, and the respiratory relations to environment are 

 factors in determining that rate. If a region of higher rate of 

 respiratory activity is determined in a cell or cell mass by dif- 

 ferential exposure to oxygen or to removal of CO 2 that region 

 is in a state of excitation as compared with other regions and 

 the relations between it and such other, regions are primarily 

 those which exist in general between excited and unexcited 

 regions. Oxygen supply and CO 2 are then not only specific 

 material factors in the relation between protoplasm and environ- 

 ment, but they are or may be quantitative factors in this relation. 

 From the first moment when differences at different levels of 

 the gradient begin to arise, the possibility of chemical or trans- 

 portative correlation arises, and as the progressive complication 

 occurs and different substances appear in the protoplasm at dif- 

 ferent levels, these possibilities become always increasingly varied 

 complex and specific. On the other hand, it is evident that 

 definite and orderly chemical or transportative correlation as 

 between protoplasmic regions rather than protoplasmic consti- 

 tuents cannot occur until definite and orderly regional differences 

 are present. The gradient is merely the first step in the develop- 

 ment and evolution of axiate pattern, and the progressive com- 

 plications and alterations of the relations and changes initiated 

 by its appearance are terminated in the individual only by agamic 

 reproduction or some other process involving dedifferentiation 

 and reorganization, or by death, and as regards its evolutionary 

 termination, even speculation is idle. 



CONCLUSION. 



It is perhaps necessary to point out that the conception with 

 which this paper is concerned, the conception 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 



