4 LIBBIE H. HYMAN AND ALBEKT E. GALIGHER 



determinations of the total metabolism of such parts might there- 

 fore give results seemingly at variance with the gradient concep- 

 tion. The gradients of particular organs or structures are not 

 necessarily the same as those of the organism in general; we know 

 of many instances of such secondary gradients. Selection of the 

 proper organism and elimination of these complicating factors 

 are therefore essential before determinations of the total metab- 

 olism of isolated pieces possess any meaning. 



On thinking over the organisms available for such experiments, 

 it is obvious that they are few in number. Planarians are usable 

 only under carefully controlled conditions. Under such control 

 it has already been shown that the carbon-dioxide production at 

 different levels of the anterior zooid corresponds with the gradient 

 conception (Robbins and Child, '20), and similar experiments 

 on the oxygen consumption are now in progress. Certain hy- 

 droids could probably be used, and experiments on them will be 

 undertaken as soon as opportunity affords. The annelids are, 

 however, by far the most suitable organisms for this kind of 

 experimentation. Among the annelids there are many forms 

 which are morphologically much the same throughout the greater 

 part of the body in the sexually immature condition. They are, 

 moreover, long enough to permit the cutting of pieces of the length 

 necessary to eliminate the stimulation of section. As far as we 

 have been able to determine, there are, in the non-sexual state, 

 no special regions having a rate of activity different from that of 

 the body in general. The susceptibility method has shown that 

 the gradient of the digestive tract is the same as that of the body 

 wall. By means of methylene blue the gradient of the nervous 

 system has likewise been determined to be of the same character. ^ 

 In these animals, then, there is every reason to predict that pieces 

 cut from different levels of the body will show differences in 

 respiratory rate which will accord with the susceptibility differ- 



- In making a previous note on the use of methylene blue for demonstrating 

 minor gradients (Jour. Exp. Zool., vol. 24, p. 55), the senior author, being at 

 that time ignorant of the fact, failed to credit Mr. John Wood MacArthur, for- 

 merly of this laboratory, with precedence in the use of this reagent, and takes this 

 opportunity of correcting that omission. 



