REGENERATION 



445 



irradiation "does not initiate cartilage dedifferentiation directly." The 

 primary effect of the irradiation, therefore, he regards as an effect upon 

 differentiation; and the extensive cartilage dissolution as a secondary 

 effect induced by the radiation but dependent upon the effect on 

 differentiation. 



In comment upon these conclusions it may be said that instead of 

 regarding the primary effect as upon differentiation it would be simpler 

 to regard it as an effect upon cell division and so upon cells with differ- 

 entiation potencies. As a result of this effect upon cell division the 



Fig. 17. — Technique for local exposure of well-grown specimens of Triton cristatus. 

 A, top view of stage with slits aa for fastening the animal as in C and B, holes b for fastening 

 the limb as in C, and opening d for the field of exposure; B, position for exposure of two 

 animals at same time; C, detail of the fastening; D, lead cover with opening / corresponding 

 to din A;E, the same in section. (From Brunst and Scheremetjewa, 6.) 



differentiation of these cells cannot proceed in the normal manner. 

 Again, it is noted that Butler's exposures were all given after the opera- 

 tion for regeneration. It would be interesting to know whether the 

 pseudoblastema would have appeared and what would have been the 

 effect upon the cartilage, if the series of the daily exposures to the rays 

 had begun several days before the operation, or if a single heavier exposure 

 had been given. The check upon subsequent differentiation of digits in 

 irradiated but uncut limbs is perhaps also referable in the first instance to 

 a check upon cell di^dsion. As this work is being continued in Butler's 

 laboratory (48), these questions will no doubt be given further considera- 

 tion now that so excellent a beginning has been made. 



The investigation by Brunst and Scheremetjewa (6) on the effects of 

 X-rays upon regeneration of the limbs in well-grown specimens of Triton 

 cristatus has shown again the inhibitive action of such irradiation when 

 given locally (Figs. 17 and 19). The hmit of an obvious and constant 

 effect was 300 to 750 r, while 15,000 r resulted in a more or less constant 

 inhibition (Fig. 18). An exposure of 3750 r showed on the whole a strong 



