404 THE BIOLOGY OF HYDRA : 1961 



er tentacles than others at 48 hours and that (b) the offspring 

 in the subclones showed the same characteristics as their parents. 

 Subclones selected on the basis of parents regenerating within 48 

 hours a normal number of tentacles — four to six — being desig- 

 nated "Typical" and those selected on the basis of parents regen- 

 erating fewer tentacles being designated "Atypical." After a week's 

 time the two types of regenerates cannot be distinguished as the 

 "atypical" hydra slowly regenerate a normal number of tentacles. 

 Only by cutting and observing at 48 hours can we distinguish the 

 typical from the atypical by gross observation. We do find differ- 

 ences in their interstitial cell patterns, though. These atypical sub- 

 clones have maintained their "atypicalness" now for two years. 

 A third intrinsic factor studied — the aging of asexual clones 



— apparently has little effect on regenerative processes. 



A fourth intrinsic factor — the effects of symbiotic relation- 

 ships (this term is used in the broad sense, including the relation- 

 ships of parasitism and mutualism) upon regeneration in hydra 



— was investigated when opportunities arose on two different 

 occasions: (a) when the microsporidial infection of our Strain I 

 (H. littoralis) clones was eradicated by the fungicide Fumidil in 

 some of the subclones; and (b) when "non-green" clones of Chloro- 

 hydra viridissima were developed by culturing organisms in the 

 absence of light for a period of time. One of the two symbiotic 

 relationships studied (that in Chlorohydra) does not appear to 

 affect the process; the other (microsporidial infection) does affect 

 regeneration somewhat — the parasitized organisms regenerating 

 more slowly than those which have been freed from parasites. 



Three types of extrinsic factors — ( a ) mechanical, ( b ) environ- 

 mental, and ( c ) chemical ( "foreign agents" ) — were capable of 

 having profound effects as measured by our gross observations. 



One mechanical factor — the severance — was found to in- 

 fluence regeneration markedly, as the site of cutting determines to 

 a greater or lesser degree the subsequent response to other varia- 

 bles. In standardizing his analytical procedures for measuring the 

 regenerative response. Dr. Ham recognized the importance in sev- 

 ering the tentacles just below the hypostome. The importance of 

 cutting close to the hypostome consistently cannot be overem- 

 phasized. In the course of later investigations, it was noted that 



