AGAMIC REPRODUCTION AND REJUVENESCENCE 149 



including both layers of the bod}--\vall, as in hydra, and in its earlier 

 stages is rounded in form and inclosed in the chitin<jus perisarc 

 which covers the stem. As development proceeds, it emerges from 

 the perisarc, undergoes elongation, and the tentacles begin to appear, 

 as indicated in Fig. 51. A later stage of development is shown in 

 Fig. 52, a fully developed hydranth in Fig. 50, //', and an old 

 hydranth bearing a medusa bud, m, in Fig. 50, //. The agamic 

 production of hydranths in this form is then a reconstitution of a 

 portion of the stem into a new hydranth. 



As regards the susceptibility of the different stages, both motor 

 activity, as in hydra, and the presence of the chitinous perisarc 

 contribute to obscure the changes in susceptibility associated with 

 the reconstitution of stem into hydranth. The susceptibilitv of 

 the early stages of hydranth development, such as //" in Fig. 50, 

 cannot be compared directly with that of stages like Figs. 51 and 

 52, because these early stages are inclosed like the stem in the 

 chitinous perisarc, while in the later stages the hydranth is naked. 

 Neither are these early stages directly comparable with such stages 

 as Fig. 50, h or //', for in the former motor activity is absent, while 

 in the latter it is fully developed. It is possible, however, to com- 

 pare the susceptibility of such a stage as Fig. 50, //', with that of 

 adjoining regions of the stem, for both are inclosed in perisarc, and 

 a comparison of this sort shows that the early bud is in general 

 distinctly more susceptible, i.e., it possesses a higher rate of metab- 

 olism and is physiologically younger than the stem adjoining it. 

 But in this case, as in hydra, the increase in rate connected with 

 the formation of a new individual is less than it would be if the 

 region were physiologically isolated and underwent development 

 at the expense of its own tissues rather than of nutritive material. 

 As it is, the bud has abundant food and grows during development, 

 while the isolated piece undergoes reduction. 



In the later stages of development the perisarc no longer enters 

 as a factor, but differences in motor activity still e.\ist between 

 different stages. At the stage shown in Fig. 51 motor activity is 

 absent or inappreciable, but the susceptibility of this stage is 

 nevertheless usually somewhat greater than that of an old hydranth. 

 like h in Fig. 50, and less than that of a younger hydranth, like 



