200 .CHARLES PACKARD 



When the exposure to radium is brief, or the amount used 

 is small, an entirely different kind of phenomenon appears. This 

 consists in a change in the division rate of cells, which is not fol- 

 lowed by any apparent abnormality. Many observers have 

 noted this effect but have made little attempt to analyze the 

 problem. In most instances they have described a retardation 

 in the rate of diidsion but occasionally they have noted an 

 acceleration. 



Richards ('14) studied the effect of X rays on the cleavage 

 of the eggs of the snail Planorbis. As X raj^s are similar to 

 the gamma rays of radium his results have a direct bearing on 

 the problem discussed in this paper. He found that brief 

 exposures produced an acceleration in the rate of cleavage, but 

 this effect was only temporary since after the first cleavage the 

 treated eggs were retarded. The greatest effect was produced 

 when exposures were made during the metaphase. During 

 the resting stage there was very Uttle response one way or the 

 other. 



The experiments described in this paper were carried on to 

 find out more exactly the conditions under which acceleration 

 or retardation may be produced by the action of radium radia- 

 tions on the fertiUzed eggs of Arbacia. 



METHODS 



In conducting experiments on the rate of cell division it is 

 of prime importance to keep the temperature of the sea water 

 constant throughout. To this end I carried out each experi- 

 ment in the following manner. Syracuse watch glasses were 

 kept in running sea water for thirty minutes to bring them to 

 an even temperature. They were then quickly dried without 

 being touched by the hands, and filled with a measured amount 

 of sea water. The Arbacia eggs, which had been freshly fer- 

 tiUzed were now added in measured quantities. It was found 

 necessary to use only a few eggs at a time for a very Httle over- 

 crowding itself changes the rate of division and produces ab- 

 normalities. One watchglass was placed under the radium tube 



