AGAMIC REPRODUCTION AND REJUVENESCENCE 151 



mediately distal to the circle of proximal tentacles (m, Fig. 50) . 

 Three stages of development of the medusa bud drawn to the 

 same scale are shown in Figs. 53-55. In the early stages the 

 medusa bud is always more susceptible to cyanide than the adjoin- 

 ing regions of the hydranth from which it arose, and its suscepti- 

 bility decreases as development proceeds, the large, fully developed 

 bud being much less susceptible than the adjoining regions of the 

 parent hydranth. These differences in susceptibility are not 

 dependent upon differences in size, for they concern primarily 

 the surface of the body. Differences in motor activity may be 

 concerned in the difference in susceptibility between the fully 

 developed medusa bud and the hydranth, but the greater suscep- 

 tibility of the bud in early stages as compared with the hydranth 

 cannot be accounted for in this way, for motor activity is present 



P 



' 6? 



53 54 



'55 



FIGS. 53-55. Pennaria tiarella: three stages in the development of a medusa bud 



in the hydranth but not in the medusa bud. Evidently the 

 medusa bud in early stages is physiologically younger than the 

 region of the hydranth from which it arises. 



But the susceptibility of young medusa buds is in general 

 distinctly less than that of young hydranths of the stage of Figs. 

 51 and 52, after emergence from the perisarc. That is, the young 

 medusa bud is not as young as the young hydranth. The medusa 

 bud arises from a more highly specialized region of the colony than 

 the hydranth bud and develops into a more highly specialized 

 zooid or individual. Apparently the reconstitution of a portion 

 of the hydranth body into a medusa bud does not carry the region 

 concerned back to so early a physiological stage as that attained 

 in the reconstitution of a region of the stem into a young hydranth. 

 This difference in physiological condition between hydranth bud 

 and medusa bud is probably the dynamic basis, or at least the 



