150 SENESCENCE AND REJUVENESCENCE 



h' in Fig. 50. At the stage of Fig. 52 motor activity is present to 

 some extent, though much less than in still later stages. This 

 stage is distinctly more susceptible than such hydranths as h in 

 Fig. 50. Here, where motor activity has begun to appear, even 

 though it is still slight, the difference in physiological condition 

 between morphologically young and old hydranths becomes dis- 

 tinctly evident. From this stage on the susceptibility decreases 

 as development proceeds, but it does not attain a constant level 

 even after the morphological form of the hydranth is fully devel- 

 oped. On a stem like that shown in Fig. 50, for example, the 

 hydranth h', which is younger in point of time than the terminal 

 hydranth h, shows in general a higher susceptibility, i.e., is physio- 

 logically younger than the latter. In spite then of the presence 

 of the perisarc in certain stages and the differences in motor activity 

 in other stages, the differences in susceptibility indicate that a 

 certain degree of rejuvenescence is associated with the agamic 

 reproduction of hydranths in Pennaria. It is still a question, 

 however, to what extent new parts which arise by budding in 

 hydroids are formed by dedifferentiation and redifferentiation of 

 old cells and to what extent by the interstitial cells which are small 

 cells lying in groups between the other cells of the body-wall and 

 which are commonly regarded as embryonic reserve cells. From 

 this point of view the apparent rejuvenescence which occurs in 

 connection with budding might be regarded as simply a replace- 

 ment of the older differentiated cells by the younger, undiffer- 

 entiated. Doubtless the interstitial cells are less highly specialized 

 than various other cells and so react more readily to the change 

 in conditions, but the very fact that they were inactive before and 

 became active in the development of the bud indicates a change in 

 their physiological condition in the direction of a higher rate of 

 metabolism. Moreover, there is every indication that at least 

 many of the specialized cells of the body -wall do take part in bud- 

 formation and actually undergo more or less dedifferentiation. 



In addition to the asexual production of hydranths, Pennaria also 

 gives rise asexually to medusa buds, which do not usually, however, 

 develop into free-swimming medusae but remain attached to the 

 parent body. These appear on the body of the hydranth im- 



