46 THE ORIGIN OF THE NERVOUS SYSTEM 



existence of graded differences in physiological condition 

 along the axes, and in the light of other data there can 

 be little doubt that differences in rate of metabolism, 

 or more particularly oxidation, play at least a very 

 important part in determining the electrical gradients. 



THE EVIDENCE FROM OXYGEN CONSUMPTION AND CARBON 

 DIOXIDE PRODUCTION 



The direct determination of metabolic activity in 

 different body regions by means of oxygen consumption 

 and C0 2 production requires the separation of the regions 

 concerned and therefore introduces various complicating 

 factors. Only in the simpler organisms is it possible to 

 maintain such separated pieces of the body in anything 

 like a normal condition, and even here the operative 

 procedure involves stimulation and may be followed by 

 depression, and cell division and growth begin at the 

 cut surfaces within a few hours after section. More- 

 over, in organisms with localized and differentiated 

 internal organs the axial gradients do not necessarily 

 run in the same direction in all organs, and a piece of 

 the body from a particular level may represent a low 

 level of the body-wall gradient and a high level as 

 regards certain internal organs, or vice versa. In 

 Planaria, for example, the mouth is near the middle of 

 the body and the activity of the alimentary tract 

 probably decreases from the mouth in both directions, 

 but in the body wall the gradient is from the head pos- 

 teriorly. If this is true, a piece of the Planaria body 

 from levels near the head represents a high level of the 

 body-wall gradient and a low level of the alimentary 

 tract, while a piece from near the mouth represents 



