ACCELERATION OF RATE OF CELL DIVISION. 355 



exposure tinder certain conditions to x-rays and radium rays, are 

 perhaps the most marked which any of the stimulating agents are 

 able to bring about. The effect of the radium rays upon division 

 rates have been studied by Hertwig, Packard, and others. Pack- 

 ard exposed Arbacia eggs to radium rays and noted that " A short 

 radiation brought about a stimulation ; while a longer one produced 

 a retard. Between these two limits there was a strength of 

 radiation which produced no noticeable effect i.e., the initial ac- 

 celeration was overcome by a subsequent retard." He argues 

 that these results were brought about because the radiation thus 

 effects the enzymes of the cells. These conclusions are confirma- 

 tory to a preliminary experiment of Lazarus-Barlow and Bonney 

 and to subsequent extensive research by Lazarus-Barlow and 

 Beckton. Since these two latter papers appeared in a publication 

 not generally accessible to zoologists in this country, it seems worth 

 while to quote the conclusions in full : 



' If radium act on ova of Ascaris inegalocephala in the resting 

 stage in quantities of the order 5 X io~ 7 mgr. and for a continuous 

 period of about 30 hours at o C., cellular division subsequently 

 proceeds at an accelerated rate. 



" Greater quantities than the above or more prolonged exposures 

 progressively retard the rate of division. 



' These effects are brought about by the action of alpha, beta, 

 and gamma rays acting together. 



" Beta and gamma rays alone (alpha rays being excluded) act 

 similarly. 



' The action of the alpha rays appears to be about one hundred 

 times as great as the action of the beta rays." 



Packard, Lazarus-Barlow and Beckton, and Mottram all agree 

 that the acceleration is greater if the exposures are made in the 

 dividing stages of nuclei rather than during resting periods. Mot- 

 tram concludes " that the animal cell, as exemplified by the ova of 

 Ascaris, is at least eight times as vulnerable to the beta plus 

 gamma rays of radium in the dividing as in the resting stage of 

 its nucleus; and, further, that this increased vulnerability during 

 division concerns the metaphase." This general ' conclusion that 

 eggs are more susceptible during the height of the division stages 



