424 



BIOLOGICAL EFFECTS OF RADIATION 



restricted and delayed as to formation of such new tissue and mouth 

 openings (Fig. 7). Eyes appeared only on the tail pieces exposed 

 5, 10, and 15 min., respectively, and their appearance was some 4 days 

 later than in controls. The individuals receiving these short exposures 

 did not show the deformations found in longer exposures and the regener- 

 ation was eventually completed. Exposures of 30 min. and longer showed 

 progressive reduction in the amount of regeneration tissue formed and 

 correlated failure to form the lost parts (Fig. 7), followed by increasing 

 deformation in later stages. The head pieces seemed slightly more resist- 

 ant to deformation than the tail pieces. 



A' 5 Min. 10 Min. I& Min. 30 Min. t Std. 2 Sid. 4 Std. 6 Std. 8 Sid. 15 Std. 



> 5 Tfje. 



7 Tge. 



Am 14. Tag nach BestrahluDg sind sSmtllche 



Tellstacke, die hCher als 30 Min. bestrahlt 



warden, deformiert. 



14 Tijc. 



Fig. 7. — Cap of regenerative tissue on head and tail pieces of controls and irradiates 

 of Polycelis nigra, exposed to radium from 5 min. to 15 hr., as the cap appears at 5, 7, and 

 14 days, respectively. K, the control. The cap is white, the old tissue shaded. {From 

 Weigand, 67.) 



In other experiments the worms were exposed and cut 1, 1^^, 2, and 

 3 days after exposure. In general this procedure resulted in progressively 

 greater injury than occurred when the cutting followed immediately 

 after exposure. In those cut at 3 days, the formation of the cap failed 

 almost completely, the concrescence and deformation appeared earlier, 

 and the mouth opening appeared less frequently. 



From one experiment made upon Planaria torva and two upon 

 P. lugiibris, for comparison with the foregoing results in Polycelis nigra, it 

 appeared that P. torva reacted much the same as P. nigra when cut 

 immediately after exposure, while P. luguhris showed greater resistance 

 to similar exposures by the formation of more regenerative tissue, by 

 later and less inhibition of regeneration, and by less deformation. 



In a series of experiments in which the worms were cut from 1}^ to 

 9 days before exposure (Fig. 8), it was shown that the cap of regenerative 

 tissue became more resistant to the irradiation as it grew older, that is, as 

 its differentiation proceeded. Complete regeneration occurred when the 

 exposure was not made until a sufficient age had been reached by the 



