CHAPTER VIII 



GRADIENTS AND FIELDS: DETERMINATION, DIFFER- 

 ENTIATION, AND DEDIFFERENTIATION 



GRADIENTS, GRADIENT SYSTEMS, AND DEVELOPMENTAL FIELDS 



THE term "physiological gradient" has been applied to spatial 

 patterns in living organisms characterized by a gradual progres- 

 sive differential in certain expressions of physiological condition. 

 That quantitative metabolic differentials seem to be the most conspicuous 

 features in early stages of many forms and that they are essential factors 

 in development is a justifiable conclusion from the data of chapters ii-vii ; 

 but concerning the particular chemical reactions and the substrate and 

 how they differ at different levels, we know little. However, if decrease 

 in concentration or amount of a certain substance or substance-complex 

 occurs in one direction along a gradient, there must be increase in con- 

 centration or amount of some other substance or substances unless there 

 is decrease in volume. A substance gradient decreasing from apical to 

 basal pole in an egg, for example, must be complemented by another sub- 

 stance gradient, different in character in the opposite direction. In many 

 eggs we find two such opposed substance gradients: the active metabohz- 

 ing cytoplasm decreases basipetally, yolk acropetally; but the resultant 

 in such cases, at least in early development, may be a single activity 

 gradient, decreasing basipetally. In some other eggs we find no direct 

 evidence of substance gradient; but a single activity gradient, also de- 

 creasing basipetally, is nevertheless present and unquestionably associated 

 with graded difference in the protoplasmic substrate. The presence of an 

 activity gradient, as determined or indicated by methods at present avail- 

 able, gives no definite information concerning quantitative or qualitative 

 character of differences in the substrate. Development as a continuing 

 series of changes is an expression of the dynamics of living protoplasms, 

 and very commonly its pattern in early stages appears to be represented 

 wholly or largely by the gradient system present. Specific or qualitative 

 material regional differences are apparently not necessarily concerned in 

 the earliest stages of developmental patterns, though there is no question 



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