DOMINANCE AND PHYSIOLOGICAL ISOLATION 307 



tinue its development unaltered when physically isolated, but it has cer- 

 tainly become so far isolated that it can develop in a way different from 

 preceding stages. 



Dominance of the primitive type may be effective in establishing a 

 gradient and later in maintaining it, and so in determining the course of 

 development of its different levels. Such a region is an inductor in earlier 

 stages and later may determine persistence of the induced development. 

 Even a part capable of self-differentiation after a certain developmental 

 stage may be incapable of continued existence after differentiation with- 

 out influence of a dominant region; muscular tissue, even in higher verte- 

 brates, is an example. 



In recent years much has been learned concerning the roles of specific 

 chemical substances as factors of physiological dominance in develop- 

 ment and of maintenance in adult life. As regards the great significance 

 and the exceedingly complex interrelations of chemical dominance, there 

 can be no question. However, most of the investigations in this field con- 

 cern advanced stages of development or functional relations in mature 

 individuals and, except for the recent work on plant hormones, are 

 largely concerned with the higher vertebrates. It has often been pointed 

 out that the earlier the stage of development the less evidence it affords 

 of specific interrelations of parts or of chemical dominance. If quantita- 

 tive gradients are the primary factors in axiate pattern, it follows that 

 chemical dominance by production, transport, and effect of specific sub- 

 stance is not the primary form of physiological dominance but is possible 

 only after different regions have become, to some degree, specifically dif- 

 ferent. The high region of a gradient may produce more of a certain sub- 

 stance than other levels, and its transport may alter the concentration 

 of this substance at other levels and so influence their condition; but this 

 is a nonspecific dominance directly related to gradient differences. A cell 

 membrane may, in a sense, dominate the cell interior by its specific perme- 

 ability to substances; but even in this case the membrane has become 

 different from the interior in consequence of exposure to an external 

 medium, and it is, in general, merely selective, not productive. 



With origin of specific differences in development of the individual, 

 chemical dominance becomes possible and with progress of differentiation 

 evidently plays an increasingly important part in determining and influ- 

 encing the further course of development, attaining its highest develop- 

 ment in the hormone interrelations of the higher vertebrates. Even in 

 these organisms, however, nervous dominance is still the chief integrating 



