REGENERA TION 415 



an elimination of interstitial cells can be virtually accomplished by the 

 technique of irradiation. 



In correlated investigations Zawarzin (71) and Strelin (62) have 

 described the gross and also the histological effects of X-rays upon 

 regeneration and budding in Pelmatohydra oligactis. The exposures, 

 given as 79 r per min. ranged from about 100 to 395 r. The gross effects 

 of this irradiation, as described by Zawarzin, included both a stimulation 

 and a retardation upon budding as well as regeneration. These effects 

 were so related to the irradiation that the stimulating effect appeared 

 immediately after exposure, the retarding effect only after an interval 

 or latent period. There was no latent period in the sense of a period 

 during w^hich no recognizable changes occurred. The total effect was 

 the sum of these retarding and stimulating factors. As would be the 

 case with temperature, there were optimum and maximum exposures so 

 that the rule of Arnd-Schulze held for X-rays in this instance. In addi- 

 tion to the primary stimulation by the rays, there appeared in later 

 stages following heavy exposures a secondary stimulation as evidenced 

 by the regeneration. The operations for regeneration resulted in an 

 increased resistance to the rays, since operated individuals survived strong 

 exposures better than nonoperated individuals. Spring hydras were 

 found to be more resistant to the rays than summer hydras. The quan- 

 titative method of estimating the rapidity of regeneration by number of 

 tentacles regenerated was regarded by Zawarzin as sufficiently accurate 

 for estimation of the biological effects of the radiation. The failure to 

 secure clear evidence for a stimulating effect of such radiations in the 

 higher vertebrates Zawarzin believed due to complexities of the physio- 

 logical reactions which mask any such response that may occur. He 

 accepted the conclusions of others that such a stimulating effect can be 

 demonstrated in plants, in protozoa, in various invertebrates, and even 

 in the lower vertebrates, as shown by the more rapid growth of axolotl 

 larvae after weak exposures to X-rays (27). 



Strelin (62), in a histological study of Zawarzin's material, found a 

 complete agreement between the occurrence and activities of interstitial 

 cells and the power of regeneration in these P. oligactis. The increased 

 rate of regeneration, which Zawarzin considered evidence for a stimulating 

 effect by the rays, was correlated by Strelin with a more rapid differentia- 

 tion of interstitial cells and hence their more rapid disappearance from 

 the ectoderm, with eventual retardation or loss of the power of regenera- 

 tion. There were no interstitial cells to be found in the later stages of 

 hydras receiving heavy exposures and such hydras had no power of 

 regeneration (Fig. 1, A and B). These heavily exposed individuals 

 showed a certain degree of degeneration which Strelin attributed to the 

 loss of functions normally performed by the interstitial cells. The most 

 significant cases appear to be those following exposures in which some 



