EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 75 



case in which the division was completed in less than forty-five 

 minutes. On the other hand, cases have been observed in which 

 fully two hours elapsed before a second cycle began. 



It is, of course, true that the rate of division can be changed 

 by varying temperature. Eggs which are kept in a refrigerator 

 will require several hours to complete a division. Similarly, eggs 

 will divide much more rapidly upon a warm day. The state- 

 ments made in the preceding paragraph, however, are based upon 

 conditions which obtain normally, or at least upon conditions 

 which are as near normal as it is possible to come in the laboratory. 



It may be conservatively stated that the range for these early 

 cleavages is from fifty minutes to more than two hours. 



If Planorbis eggs are exposed to the rays during the resting 

 stage between two mitoses the results are less marked than if the 

 exposure is during the progress of the mitosis. 1 It is quite 

 certain from the later behavior of eggs so treated that no ex- 



Incomplete observations on Physa eggs seem to indicate a similar range of time 

 values for the cleavage divisions in that form. 



.(This observation and those included in the succeeding footnotes form only a 

 part of the data taken in this investigation. The arrangement here and the number 

 of the various experiments indicate nothing more than convenience for reference. 

 The experiments, of which there were many more than are here given, were not 

 made in this order; these are chosen merely as examples bearing on the points under 

 discussion.) 



1 (2) At the beginning of this exposure, the eggs had just finished the first 

 cleavage and their nuclei were resting. They were exposed to the X-rays for six 

 minutes. At the end of this time there were no visible effects. Thirty-five minutes 

 later the second furrow made its appearance and in one hour and fifteen minutes 

 after the exposure, the second division was complete. Two and one-half hours 

 were required for the next division, a much longer time than normal. 



(3) The eggs used in this experiment had been observed during the progress 

 of the first cleavage division, and were exposed for six minutes at the time when the 

 blastomeres were most widely separated and the nuclei were resting. The exposure 

 had no visible effect, and the eggs apparently did not depart from their normal 

 course. One hour and fifty minutes, however, were required for the next cleavage. 

 The third cleavage consumed one hour and fifteen minutes, and in two hours more 

 the sixteen cell state had been reached. 



(4) At the time of exposure the first cleavage furrow had separated the blasto- 

 meres fully. The exposure lasted three minutes and produced no effect except that 

 ten minutes after it had begun the blastomeres had flattened against each other. 

 One hour and ten minutes after the exposure, the second furrow made its appearance 

 in some of the eggs. Not all had progressed equally, some having nearly completed 

 the second division. The first quartette had been given off and its division begun, 

 at the end of one hour and forty five minutes more. 



Other cases might be cited. 



