342 CHARLES PACK.\RD 



variation in the responses of the Bacteria. Obviously, those 

 cells which are injured contain substances which absorb the rays, 

 while those which are uninjured allow the rays to pass through 

 unchanged. The factors which determine the power of absorp- 

 tion of materials are not well known. ''The absorption of beta 

 rays is an atomic phenomenon and is not affected by the chemical 

 combination of the atoms. Such a relation appears to hold 

 generally for all types of radiations emitted by radioactive sub- 

 stances" (Rutherford). Until more is known on this point it 

 will be impossible to predict what effect a given exposure will 

 produce. These facts throw no light on the nature of proto- 

 plasm, but accentuate the point that the protoplasm of one type 

 of cell differs from that of another cell. 



Disregarding these differences, it may be said that a weak 

 radiation accelerates, and a stronger one retards cell division. 

 Acceleration is not followed by any abnormality. A careful 

 study of this point has been made by Lazarus-Barlow and 

 Beckton ('13) who used exceedingly small quantities of radium 

 on Ascaris eggs. Tests made upon many thousands of eggs 

 showed that cell division is accelerated when the exposure is 

 not too prolonged. After an optimum length of exposure the 

 rate of cell division is gradually retarded. I have shown that 

 sea urchin eggs are accelerated by a weak stimulation. The 

 kind of rays seems to make no difference with the result. In- 

 asmuch as the gamma rays are very penetrating, and therefore 

 are not absorbed to any extent, they are the 'weakest' and must 

 be allowed to act for a long time before they can produce any 

 effect. The beta rays are more readily absorbed and will pro- 

 duce an acceleration if not allowed to act for too long a time. 

 If the alpha rays are allowed to act in unison with the other 

 types, acceleration will follow after a few seconds' exposure. 

 These rays are about one hundred times as effective as the beta 

 rays, and the beta rays are more effective than the gamma rays 

 in the same proportion. These figures correspond roughly to 

 the respective powers of ionization of the rays. 



Retardation follows a moderate radiation of the beta rays. 

 This effect is not peculiar to them, for if they are mixed with 



