8O A. RICHARDS. 



with the normal, it appears that only a small fraction of the time 

 usually taken is consumed under the conditions of the experiment. 



It has been difficult so far to get much information concerning 

 the phase of acceleration. It is passed very quickly; in the last 

 cases cited scarcely more than a minute was consumed, and in no 

 instance was more than ten minutes of exposure necessary to 

 bring about the result. It seems that there is a minimum time 

 in which a cell division in the early cleavages of the egg of 

 Planorbis can take place actually something more than a 

 minute and that the stimulation by exposure to the rays need 

 be but very short in order to reduce the time from normal to this 

 minimum. 



Aside from their bearing on the effect of X-rays on the living 

 organism, the facts connected with the shortening of the time 

 necessary for mitosis have considerable interest from their 

 bearing on the questions of cell mechanism. 



The first effect, then, of exposure on the rate of cleavage is to 

 stimulate greatly whatever mitosis may be in progress and to 

 hurry the cells into the resting stage. 



The effect of exposing a cluster of eggs not all equally advanced 

 forms an interesting corollary to the observations on the induced 

 acceleration of individual eggs. The usual conditions in Plan- 

 orbis as in most forms, is that all the eggs in a cluster are in the 

 same stages of development. However, there sometimes occurs 

 a variation from the general condition and of the two dozen eggs 



thirty minutes yet were consumed before the divisions were completed. An hour 

 and a half later the eggs were in the i2-cell stage. 



(18) Eggs in the four cell stage were exposed twenty seconds. Five minutes 

 elapsed before they were examined but at the end of that time the first micromere 

 quartette had been nearly, if not completely, divided off. An hour later the second 

 quartette had appeared and in another thirty minutes the first quartette had 

 divided. Seven minutes later a second exposure of twenty seconds was made, at 

 the end of which in at least part of the eggs the second quartette had divided. It 

 was difficult to see cell boundaries after this stage, but the third quartette could not 

 clearly be seen until after another hour and a quarter had elapsed. 



(19) The eggs of the cluster were well along in the second division at the time 

 of exposure, which lasted twenty seconds. When they were examined (as soon as 

 possible after the exposure) the division seemed to be entirely finished. Thirty five 

 minutes later the first quartette was given off, and was followed by the second after 

 another thirty five minutes. A second exposure of twenty seconds was now given; 

 at the end of it, in part of the cells at least, the first quartette had divided. An 

 hour and a quarter elapsed before the end of the next division. 



