ROBERT E. EAKIN 405 



marked variations occurred in rate of regeneration in hydra from 

 a uniform subclone in the same dish of rephcates. In an effort to 

 trace the source of this \ ariabihty, many factors were considered 

 and examined. Among these was a comparison of the regenera- 

 tion of some hydra that were cut while in the normally contracted 

 state and others in the normally stretched state. It was found 

 that the hydra cut in the contracted state regenerate at a slower 

 rate than those cut while in a stretched state. To determine whether 

 this was due to the removal of more tissue or to the occurrence 

 of a larger wound in the hydra cut in the contracted state, some 

 of the stretched hydra were cut just behind the hypostome and 

 some at the mid-stomach region (midway between the hypostome 

 and the budding region). Since the hydra were stretched while 

 cut at both locations, the size of the wound was consistent but 

 the amount of tissue remoNcd varied. By referring to Table 1 again, 

 we can see the differences that could result if the cutting were 

 not accurately done — or if the organisms were not in a fully 

 extended state at the time of cutting. 



The variation in regeneration rate caused by cutting the differ- 

 ent hydra groups at the two locations is not the same in all the 

 hydra tested. The ChJorohijdni and Hydra of Strain I were only 

 slightly retarded in regeneration rate when cut at the mid-stomach 

 region, whereas the regeneration rate in Strains II, III, and IV 

 was quite retarded. The strains of hydra whose regeneration rate 

 is decreased by cutting at the mid-stomach region also regenerated 

 fewer tentacles than control hydra cut at the hypostomal region, 

 a finding that can be explained by the observations made on 

 interstitial cell differentiation and distribution to be discussed later. 



As has been reported previously (2, 4), adding a foreign chem- 

 ical or varying a natural environmental factor (pH, inorganic ion 

 concentrations, osmolarity, temperature, or the adequacy of nutri- 

 tion) can affect the regenerative response. In re-examining these 

 factors in the present study, two new observations were made: 

 ( a ) that often the qualitative effect of a physical or chemical agent 

 will depend upon the exact site of severance; and (b) that, as 

 is shown in Table 3, the temperature at which the stock clones 

 have previously been maintained can affect the rate at which the 

 experimental organisms later will respond under the standard tem- 



