ROBERT E. EAKIN 407 



the extent of fasting they have undergone. Also, since these are 

 small contracted spherical hydra, it is very difficult to remove 

 the tentacles and hypostomes accurately at the desired site. 



Observations on all these and other variables impressed us 

 even more emphatically with the fact that, before meaningful 

 studies can be made on the action of any type of chemical or 

 physical treatment "foreign" to the natural environment, one must 

 have used extreme care in controlling all the factors just discussed. 



Also, postulates advanced concerning mechanisms controlling 

 the rate and extent of regeneration should offer some explanation 

 of these differences observed in the regenerative response. 



CHEMICAL INHIBITION OF REGENERATION 



The second aspect of our hydra research program which I 

 wish to discuss concerns the effects of chemical agents upon the 

 normal regenerative processes in well fed organisms. In order to 

 find agents which would have selective action upon the regen- 

 eration process without materially affecting the normal mainten- 

 ance and budding activities of the organism, hundreds of com- 

 pounds having physiological or pharmacological effects on other 

 types of life were screened, and from these a few compounds 

 having marked effects were selected for more intensive study. 



From the known physiological roles of the compounds that 

 inhibit regeneration and alter tentacle number, the nature of a 

 number of the processes involved in regeneration can be implied. 

 In order to gain even more information concerning the mode of 

 action of these compounds, the organisms were exposed to them 

 for short intervals at different periods during the regenerative proc- 

 ess. It was found that a four hour exposure at the proper time was 

 adequate to obtain the effects of most of the compounds. We also 

 found that different compounds acted at different times ( Fig. 1 ) . 



The results of most of these studies were published several 

 years ago along with a discussion of possible mechanisms involved 

 (2). Most of these studies were completed before we realized the 

 extent to which the factors just discussed were influencing the 

 regenerative response. Another drawback, realized at the time 



