20 INDIVIDUALITY IN ORGANISMS 



As following chapters will show, there is reason for 

 believing that what we call an axis in the organism 

 represents the general course and direction of a gradient 

 in rate of metabolic reactions, the rate of reaction being 

 highest at one end, or in a certain region, and decreasing 

 from this point in the direction in which we conceive 

 the axis to extend. Moreover, the physiological and 

 structural order along any axis is definitely related to this 

 gradient. If all organic axes are fundamentally meta- 

 bolic gradients we may call the region of highest rate 

 in any axis the apical region or end, the region of lowest 

 rate the basal region or end, while other intermediate 

 regions may be distinguished as more or less apical or 

 basal, and opposite directions in the axis as respectively 

 apical and basal directions. From this point of view 

 apical and basal regions of radially symmetrical whole 

 organisms are merely the apical and basal regions of the 

 major axis of such organisms and so the most conspicu- 

 ous or most widely separated apical and basal regions 

 of the body, but not fundamentally different in their 

 dynamic significance from the corresponding regions of 

 other axes. In the following pages it will often be con- 

 venient to use these terms, "apical" and "basal," in 

 this general way for bilaterally as well as for radially 

 symmetrical forms. 



