394 THE BIOLOGY OF HYDRA : 1961 



more likely in our case, because selection pressure has not been 

 sufficiently severe to provide a replacement regimen for those cells 

 and tissues which are relativel)' well shielded from accidental dam- 

 age or loss (29). 



ACKNOWLEDGEMENTS 



The author wishes to acknowledge the collaboration of Drs. 

 David Brandes and Mary Ann Brock in many of these studies as 

 well as the constant and invaluable assistance in all phases of this 

 work bv Mr. Malcolm Gee. 



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