THE AXIAL GRADIENTS IN HYDROZOA. 40! 



evidence that distal levels have a higher metabolic rate than 

 proximal levels. 



(d) Difference in the rate of regeneration of apical and basal 

 pieces. Work upon this point constitutes the bulk of the paper 

 and is summarized under the subsequent heads. 



3. Apical halves of the stem of Tubularia regenerate oral 

 hydranths markedly faster than basal halves. In cutting such 

 pieces it is essential to discard the original hydranth and the 

 first millimeter or two of the stem; the remaining stem is then 

 cut into two equal halves. The difference between such halves 

 has been demonstrated by: 



(a) Mass experiments in which all of the apical halves have 

 been placed in one dish, the basal in another. In such cases, 

 the number of apical pieces which have regenerated oral hy- 

 dranths is nearly always in excess, rarely equal to, and never 

 less than the number of basal pieces which have regenerated. 



(b) Individual experiments, in which the number of hours 

 required for the emergence of the oral hydranth on each piece 

 was recorded. The apical pieces regenerated oral hydranths 

 first in 91 per cent, of the cases (122 pairs of pieces observed). 



4. Apical pieces regenerate on the whole more rapidly than 

 basal pieces, even when the latter are twice as long as the apical 

 pieces. Such apical pieces must not however be less than 5 mm. 

 in length. In pieces over 10 mm. in length, length has very 

 little effect upon the time of regeneration; in pieces less than 

 10 mm. in length, the longer pieces regenerate faster than shorter 

 ones having their distal ends at the same anterior level but this 

 effect of length is not sufficient to overcome the influence of 

 level except in very short pieces (under 5 mm.). 



5. The difference in rate of regeneration of apical and basal 

 pieces which exists under normal condition can be somewhat 

 reduced by using stems bearing branches and cutting the apical 

 piece above the branch and the basal piece below the branch. 

 Since the first branch marks the limit of the Tubularia individual, 

 the apical pieces above such branches are really the basal regions 

 of the principal Tubularia individual, and the basal pieces below 

 the branch are near the apical end of the second individual. 

 In consequence of these relations, the difference between the 



