THE AXIAL GRADIENTS IN HYDROZOA. 



199 



a single exception, show a secondary gradient or gradients in the 

 stalk, and this secondary gradient extends further acropetally 

 than in the other two species. In the first place, it is much more 

 difficult to obtain attached quiescent individuals of H. oligactis 

 for observation, for no matter how firmly attached and inactive 

 they may be preceding the addition of the agent, most of them 

 detach themselves either when the water is drawn off or when the 

 agent is added. In many cases attachment occurs again in the 

 solution within a few moments but is it preceded by more or less 

 motor activity of the stalk. Moreover, in the individuals which 

 remain attached the stalk apparently always undergoes some 

 contraction. 



In fact, probably the only completely successful attempt to 

 obtain an attached quiescent individual for susceptibility deter- 

 mination is the case of the single exception mentioned above. 

 This was a small animal, only recently separated from the parent 

 body, which remained attached and quiescent during the whole 

 period of observation and showed only the primary basipetal 

 gradient (Figs. 43-46) like attached quiescent individuals of H. 

 viridissima and H. vulgaris. Since this animal was not full grown 

 and not far beyond the bud stage, and since the later bud stages 



47 



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