196 C. M. CHILD. 



polarities, but some of them are certainly, and all of them may be 

 new axes. The fact is of interest that in such forms adventitious 

 stolon-axes may arise from any part of the free surface, but the 

 direct origin of a hydranth-stem axis has been observed only 

 from the most exposed portion farthest from the surface in 

 contact (Figs. 22, 23). 



The fact that the stolon axes usually appear first in acclimation 

 and recovery and only later transform into, or give rise by 

 budding to hydranth-stem axes confirms the conclusion of an 

 earlier paper (Child, '23). In that paper it was shown that apical 

 regions of various hydroid colonies can be transformed into 

 stolon axes by slight degrees of inhibition or depression and that 

 the stolons can again give rise to hydranth-stem axes with 

 acclimation to, or recovery from the action of the agent. Such 

 apical transformations do not represent reversals of polarity, but 

 rather simply a depression of the gradient to a lower physiological 

 level. In Lund's experiments on Obelia with electric current 

 (Lund, '21) the position of the piece with respect to the electrodes 

 merely determines whether the gradient arising at a given end 

 shall be more or less inhibited and develop as a stolon or whether 

 it shall attain the higher physiological levels characteristic of the 

 hydranth-stem axis. Since axes, either stolon or hydranth-stem, 

 very commonly arise from both ends of pieces without action of 

 the electric current, it is evident that the current is not necessary 

 for the determination of these polarities, even though it does 

 determine their character as stolon or hydranth-stem axes and 

 may, when acting, assist in determining the gradient at one end 

 or the other of the piece. In any case the effects of the current 

 are in no way specific as regards determination of the two sorts of 

 axes, for, as I have shown, essentially the same results can be 

 obtained with various chemical and physical agents in both 

 hydroid colonies and developmental stages. Apparently all that 

 is necessary to transform a hydranth-stem gradient into a stolon- 

 gradient is a slight degree of inhibition, and the reverse transfor- 

 mation is accomplished by environmental changes in the reverse 

 direction. In Lund's experiments with Obelia the anode ap- 

 parently acts to some extent as an accelerating, the cathode as an 

 inhibiting agent. 



Physiologically the stolon does not represent simply the basal 



