82 A. RICHARDS. 



so far as now determined, without regard to which particular 

 cleavage is in progress. The whole question of the effect of the 

 rays so far as the rate of division is concerned is not one of the 

 state of progress of the egg in cleavage but one of the mitotic 

 cycle. As long as the exposure is made at the same relative time 

 in the mitosis it makes no difference with which of the early 

 cleavage divisions the experiment begins. It is as though the 

 energy of the eggs was used up upon exposure to the rays in 

 hurrying the eggs into the resting condition and that continuously 

 more and more time is required to raise the cell to the point where 

 it can again divide. Or perhaps it may be plausible to explain 

 the result on the grounds that the X-rays have a double effect: 

 first a stimulative effect, and second, a very injurious inhibitive 

 effect. The former effect is produced during what may be called 

 the latent period of the latter. That there is some particular 

 factor concerned more than mere stimulation is seen from the 

 experiment described below in which a second phase of accelera- 

 tion and the second phase of depression was obtained by a second 

 exposure. The second stimulation was less and the depression 

 more rapid than the first. Now, if the stimulation were all that 

 had taken place, as for instance in a muscle-lever experiment 

 where the results of simple stimulation are obtained, the second 

 stimulus should have been quite as effective as the first; such 

 however was not the case. 



It has already been stated that very little effect results from 

 exposing the eggs in the resting stage; however, there is some 

 evidence for thinking that such an exposure causes a depression 

 in the rate of division after it. The data at hand on this point 

 are not as positive as desirable, but it seems to indicate that 

 conclusion. In two or three hours after the exposure the divisions 

 became slower and slower as they would have done (but to a 

 much less degree) if the egg had been exposed during mitosis. 



There is one case forming an apparent exception to the general 

 observations as stated above. If the exposure occurs during the 

 first maturation division, the depression does not set in until the 

 first cleavage mitosis. 1 The first and second maturation divisions 

 take place with considerable rapidity and show the effects of 



1 Compare experiment (9). 



