6o INDIVIDUALITY IN ORGANISMS 



distinguishable. Alteration or even reversal of certain 

 gradients during development in some cases makes it 

 necessary to distinguish between the primary gradients, 

 existing at the beginning or in the early stages of devel- 

 opment, and the secondary gradients, which arise by 

 alteration of the primary. 



The primary relations between the most conspicuous 

 metabolic gradients and the chief axes of the individual 

 is briefly as follows. The major axis is represented by a 

 gradient in which the apical region is always primarily 

 the region of highest, and the basal, that of the lowest, 

 rate of reaction. Stated in different terms, the region 

 of highest metabolic rate in this gradient always gives 

 rise in development to the apical region or head of the 

 animal, the region of lowest rate to the basal or posterior 

 end. In radial gradients the region of highest rate may 

 be either peripheral or central according to the character 

 of the radius. In bilaterally symmetrical animals the 

 relations differ in different cases. In at least most 

 bilaterally symmetrical invertebrates the median ventral 

 region is primarily the apical region of the minor body 

 axes, and from this region gradients of decreasing rate 

 extend laterally and. dorsally . In the vertebrates, on the 

 other hand, the median dorsal region is primarily the 

 apical region, and gradients of decreasing rate extend 

 laterally and centrally. The fact must be emphasized 

 that these are the general and primary relations and that 

 they may be altered in various, but always orderly and 

 definite, ways during the development of the individual. 



These facts indicate very clearly that the chief axes 

 of the animal body are represented dynamically by 

 metabolic gradients and that each organ or part arises 



