PHYSIOLOGICAL GRADIENTS. 1 67 



any case the axial gradients in potential indicate the 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. 



Oxygen Consumption and Carbon Dioxide Production in Rela- 

 tion to the Gradients. The direct determination of respiratory 

 activity in different body regions by means of oxygen consump- 

 tion and CO 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 the section. 

 Moreover, 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 

 Plan-aria 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 posteriorly. 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 the reverse condition. Un- 

 less we can eliminate one or the other of these gradients the 

 oxygen consumption and CO 2 production of such pieces is not 

 likely to give us any very definite information as regards either 

 gradient. In this case, however, we can bring the alimentary 

 tract into a more or less quiescent condition by starvation, and 

 then we find that a gradient in oxygen consumption and CO 2 

 production does exist in the body wall, the high end being at the 

 head with a second rise in the region of the posterior zooid. The 

 data along this line are as yet mostly unpublished, but one paper 

 on CO 2 production in Planaria has already appeared (Robbins 

 and Child, '20) and work on oxygen consumption has been done 



