SUSCEPTIBILITY IN HYDROIDS. 1 17 



dominant and displaces the former terminal hydranth. In 

 this connection it is of interest to note that the Obelia bud grows 

 for some time before it differentiates into a hydranth (Fig. 10). 

 During this period of growth it is the seat of intense physiological 

 activity and shows a high susceptibility, which decreases as the 

 hydranth develops, and it is during this period that it displaces 

 the former apical hydranth. In Pennaria the bud differentiates 

 into a hydranth at once with but little growth in length (Fig. 

 7). That these differences in behavior are associated in some 

 way with the differences in growth form cannot be doubted, and 

 it seems probable that they may be the determining factor. 

 Since dominance in axial relations has been shown to be funda- 

 mentally associated with differences is intensity of metabolic 

 activity the intense growth activity of the bud of Obelia and 

 similar forms probably enables it to dominate and displace the 

 former apical bud which has completed its period of rapid 

 growth and is undergoing differentiation into a hydranth. 

 In Pennaria, on the other hand, where the bud differentiates 

 with very little growth, the intensity of its activity decreases 

 from an early stage and is apparently less at any stage than in 

 the Obelia bud, consequently it is unable to alter existing axial 

 relations and becomes a subordinate member of the complex. 

 This interpretation is offered merely as a suggestion, based on 

 the data at hand. Whether it is correct or not can probably be 

 determined only by experimental work such as has been done 

 with plants on control and modification of the relations of domi- 

 nance and subordination by inhibition or removal of certain 

 apical regions, but such work offers difficulties at present. 



The necessity of using freshly collected material for the study 

 of the regional differences in susceptibility, the colony gradients, 

 as well as for other work on susceptibility may once more be 

 emphasized. In colonies kept in the laboratory over night the 

 regional differences have often disappeared or are reversed in 

 the colony or branch. 



That there is a real difference in physiological condition in 

 different regions of the colony is indicated by the fact that the 

 medusa buds in Pennaria and Bougainvillea and the gonozooids 

 in Obelia appear first in the most basal regions of the most basal 



