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THE AMERICAN NATURALIST [Vol.LVI 



in a region of rapid oxidation certain substances might 

 be entirely oxidized as rapidly as they are formed, while 

 in a region of slower oxidation they might accumulate 

 as part of the structure. 



Bilateral organisms follow a law of antero-posterior 

 development. Differential exposure having determined 

 which region shall lead and having thus set the rate of 

 activity of the successive regions, continuing differential 

 relations of the environment maintain the various levels 

 of the gradient, under the domination of the head-end, at 

 their respective rates of activity. While this is the 

 normal course of development, it may be greatly altered 

 by experimental methods; the original gradient, partic- 

 ularly in lower organisms, may be obliterated and a new 

 one engendered. The latter under certain conditions may 

 even be made to arise at right angles to the original axis. 

 Change of gradient may be readily observed, for example, 

 in the regulatory development of isolated pieces of many 

 planarians. Moreover, the remarkable capacity for self- 

 differentiation possessed by isolated parts taken at dif- 

 ferent levels of the body in such forms, shows that posi- 

 tional relations of the constituents of the regenerating 

 mass, rather than cellular specificity, determine what 

 structures shall arise in a given location. 



Since many species, including representatives from all 

 the chief phyla of animals, show differential susceptibility 

 at some stage of their development, when subjected to 

 the action of various external agents such as alcohol, 

 anaBsthetics or potassium cyanide, these agents may be 

 used to bring about shifts of gradient, under-development 

 or over-development of certain parts, or a remodelling 

 of the organism or of various regions of it. Inasmuch 

 as any one of several agents may bring about the same 

 result, it is manifest, again, that quantitative external 

 conditions are the factors which initiate and thereby 

 determine the fundamental orientations and specializa- 

 tions of the parts of an organism. 



As development progresses in the more complex organ- 

 isms, again through differential stimulation, secondary or 

 ''symmetry" gradients may also become established. 

 For example, each limb region of a vertebrate becomes 

 a subordinate system with its own internal correlations. 



