EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 87 



seconds is almost as effective. Obviously, if rise in temperature 

 were the cause of the more rapid division, a long exposure would 

 give a more rapid cleavage than a very brief radiation. 



Recently the writer has been carrying on some experiments, 

 the results of which will be published in another place, to ascertain 

 the effect of X-rays upon certain enzymes. The general con- 

 clusion drawn from these experiments is that the activity of 

 the enzymes in question is increased somewhat by a weak expo- 

 sure, but decreased by a stronger radiation. In the light of the 

 study on cell division here reported and especially of such obser- 

 vations as those of the effects of the stronger radiation on Physa 

 eggs, the suggestion of a possible relation between these two sets 

 of phenomena forces itself upon one. In a late paper, Packard 

 suggests "that radium radiations act indirectly on the chromatin 

 and the protoplasm by activating enzymes." This is not un- 

 likely the case, in the writer's opinion, and is in harmony with 

 the observations here presented, for the effects of radium rays 

 appear to be comparable only to those of weak X-rays. It is 

 of course by no means clear how the activation of the enzymes 

 takes place. 



SUMMARY OF THE OBSERVATIONS ON EXPERIMENTS. 



1. The eggs of Planorbis require normally from fifty-five 

 minutes to about two hours to complete a division (up to the 

 twenty-four cell stage). In no case has a division been observed 

 to occur in less than forty-five minutes. 



2. By exposure to X-rays during the resting stage of the nucleus 

 only a very slight stimulation may be produced. 



3. Exposure during the early part of the formation of the 

 mitotic spindle is most effective. 



4. The first effect of exposure upon the rate of cleavage is to 

 stimulate mitotic activity, to bring on a period of hyperactivity. 

 Usually at the end of the exposure the division has been completed, 

 and the cells hurried into the resting stage. 



5. Only a very short stimulation is necessary to produce this 

 acceleration. 



6. Following the phase of acceleration a phase of depression 

 sets in; the end result is to retard greatly the development of the 

 egg- 



