THE AXIAL GRADIENTS IN HYDROZOA. 393 



basal pieces may precede the middle pieces is to be expected on 

 the basis of what has already been said. It should be obvious 

 that in stems as long as 30 mm., the length required for these 

 experiments, the basal regions must already be more or less 

 physiologically isolated as new individuals whether branches are 

 present or not. Therefore it may be expected that at least some 

 of these basal pieces will regenerate more rapidly than the 

 middle pieces. It is furthermore to be remarked that the 

 metabolic gradient is steepest near the hydranth and gradually 

 diminishes in slope down the stem; and it has never been claimed 

 by us that any marked axial difference exists along the basal 

 parts of the stems of hydroids. Indeed, we believe that in many 

 cases the gradient has disappeared in these basal regions, as 

 shown by the tendency for such levels of the stem to produce 

 numerous adventitious buds, irregularly arranged, while in the 

 more distal levels of the stem, bud formation proceeds in a very 

 definite and orderly manner. 



4. The Effect of Depressing Agents. It has been pointed out 

 by us on numerous previous occasions that a certain relation 

 exists between metabolic rate and depressing agents, such that 

 regions of higher metabolic rate are more affected by depressing 

 agents than regions of lower metabolic rate. If this general 

 statement is correct, and various lines of evidence establish its 

 accuracy, then it should be possible to reduce, eliminate, or 

 reverse the differences in rate of regeneration that normally 

 exist between apical and basal pieces. This is the case. Only 

 two depressing agents were employed, ethyl ether and potassium 

 cyanide. Apical and basal pieces of equal length were cut in the 

 usual way and both exposed to the same concentration of these 

 substances, made up in sea-water, for a certain length of time. 

 The pieces were then thoroughly washed in several changes of 

 sea-water and completed their regeneration in normal sea-water. 



These experiments are presented in Tables IX. and X. In 

 Table IX. are given the results of all the mass experiments 

 performed with cyanide. They were performed in June, at 

 room temperature, except number 22, which was placed in the 

 refrigerator later. The concentration of cyanide used and the 

 number of hours during which the pieces were exposed to it are 



