304 THE BIOLOGY OF HYDRA : 1961 



times greater than that observed in normal specimens or than that 

 reported as the maximum height for this species in nature. Oth- 

 er experiments, but not all, showed evidence of an aging factor 

 inhibitory to growth. These studies are being continued. 



REGRESSION AND REPLACEMENT OF HYDRANTHS 



In all thecate hydroids which have been examined hydranths 

 are short-lived; they regress and are resorbed after about one week 

 (3). In Figure 4, for example, it may be noticed that there is only 

 a pedicel at the location, lowest left, where the oldest hydranth 

 "ought" to be; it had regressed. In this symposium. Dr. Strehler is 

 presenting much of our information ( 12 ) concerning this regres- 

 sion-replacement cycle and its implications for the understanding 

 of aging. 



When regression occurs, the materials of the hydranth go back 

 into the colony and are available as nutrition for further growth, 

 a point which has been proved by Berrill ( 1 ) and Nathanson (11). 

 [See comment by Crowell in the discussion of the paper by Streh- 

 ler in this symposium (p. 396).] 



In contrast with thecate species athecate hydroids do not re- 

 gress, so far as we know, except under adverse conditions. We have, 

 for example, records of Cordylophora hydranths which lived for 

 more than three months even when food was limited and growth 

 was almost at a standstill ( not previously reported ) . 



RECONSTITUTION EXPERLMENTS 



Hydroid tissues can be dissociated mechanically giving tiny 

 clumps of cells, which can be pushed together into a loose mass. In 

 both thecate and athecate species these clusters reorganize them- 

 selves into a double-layered hollow sphere with epidermal cells on 

 the outside, endodermal cells inside. Up to this point thecate and 

 athecate tissues are similar in behavior. The subsequent events 

 differ strikingly and emphasize in a different way the contrast be- 

 tween the two groups in the manner in which a hydranth develops. 



