EFFECT OF X-RAYS ON RATE OF CELL DIVISION. 79 



undergoing mitosis at the time of the exposure is very much less 

 than in the case of a cell dividing under more normal conditions. 

 Thus, of the effects induced in the egg by exposure to X-rays, the 

 first takes the form of a marked increase in the activity of the 

 egg, causing a phase of acceleration. This effect was first ob- 

 tained after eggs had been exposed ten minutes, when it was 

 noticed that divisions had actually been completed in cells where 

 only a spindle was to be seen at the time the exposure began. 

 That is, during an exposure of ten minutes there had been accom- 

 plished a complete process which never under normal conditions 

 had been observed in this form to occur in much less than an 

 hour. I have repeated this observation from January to June 

 on many experiments and have obtained the result without 

 variation. Whenever an egg of Planorbis in any cleavage up to 

 the sixth, farther than which it is not practical to carry on ob- 

 servations on the living egg, is exposed to X-rays any mitosis 

 which may have been started is hastened to its completion, and 

 in almost every case that state has been reached by the time the 

 egg can be taken from under the tube and examined under the 

 microscope. 



Subsequently, I have reduced the length of the stimulation to 

 six minutes, five minutes, and three minutes without noticeable 

 difference in the result. In each case the mitosis (both nuclear 

 and cytoplasmic divisions) in question was nearly if not quite 

 completed at the end of the exposure. Even shorter exposure 

 than this will bring about the result more or less completely. 

 I have exposed the eggs as short a time as twenty seconds and have 

 found the phase of acceleration almost as marked although the 

 cell division would not be fully completed at the end of the ex- 

 posure. Thus it is seen that a very short exposure only is neces- 

 sary to induce the acceleration. 1 Comparing this induced result 



1 Compare experiment No. 16. (17) At the time of exposure the eggs of this 

 experiment were in the early stages of the second cleavage division, but were not 

 at all in the same stage of advancement. (Some had not quite completed the first 

 division.) The exposure lasted 20 seconds, and at the end of it the eggs were 

 examined as quickly as possible. Many had almost completed the second cleavage 

 and others had passed well into it. Only a few, which were probably in the resting 

 condition at the time of the exposure, showed no effects. Fifteen minutes later 

 nearly all had completed the second division and their nuclei were resting. Forty 

 five minutes later the beginning of the first micromere divisions were visible but 



