298 PATTERNS AND PROBLEMS OF DEVELOPMENT 



formation of different products. Differences in concentration of electro- 

 lytes and t^heir ions must also occur at different gradient-levels. Electric- 

 potential differences are apparently characteristic of gradients. Different 

 rates of metabolism involve differences in enzyme activity. Present knowl- 

 edge of molecular constitution of proteins and the part which they play 

 in enzymes suggests that many of them may be extremely sensitive to 

 differences such as these and that positions and relations of groups in a 

 molecule may differ at different levels with resulting difference in reac- 

 tions. The chain of reactions concerned in oxidations is probably not 

 the same with different concentrations of reacting substances. Certain 

 substances tend to concentrate in regions of greater or less surface energy. 

 These and doubtless many other factors may be concerned in originating 

 differentiations at different gradient-levels. Relation and interaction be- 

 tween levels appears also to be an important factor in differentiation. 

 There is no theoretical difficulty as regards origin of specific or qualita- 

 tive differentiations at different levels of a primarily quantitative gra- 

 dient. In fact, it is difficult to believe that such a gradient can remain 

 without some differentiations for any considerable time. The progressive 

 increase in specificity of particular parts so generally characteristic of 

 development also suggests that the primary pattern may be without re- 

 gional specificity. If a gradient extends over more than a single cell, cells 

 along its course represent different levels, and these differences provide a 

 basis for difference in gene action, certainly an essential factor in differ- 

 entiation. The character of metabolism in a gradient within a single cell 

 is undoubtedly also determined by interaction between nucleus and cyto- 

 plasm. According to this conception, determination and differentiation 

 of parts are earlier and later stages of a continuous series of changes: the 

 primary pattern which initiates these changes and determines their or- 

 derly relation along a physiological axis is a gradient in which differences 

 in rate of metabolism constitute the effective factor. 



The assumption that axiate developmental pattern consists primarily 

 of a static structure or of localized specific substances seems to involve 

 confusion of the dynamic and material aspects of living protoplasms and 

 of development. Developmental pattern appears primarily as an activity 

 pattern with localization of specific substances and morphological struc- 

 ture as a result. Investigation of organization and development has been 

 largely in the hands of those with morphological, I'ather than physiologi- 

 cal, training; and viewpoint and theories of development have usually 

 been based on embryonic development alone. This situation is perhaps 



