78 A. RICHARDS. 



Variations in the results of exposing the eggs are probably to 

 be explained largely on the basis of this maximum and minimum 

 capacity for response. 1 In other words, if the exposure is made 

 during a resting stage, the minimum stimulation results; if it 

 is made during the period of the early spindle, the maximum 

 stimulation is obtained; but if it is made between these two 

 extremes, the result is neither maximal nor minimal and there is 

 only a partial degree of stimulation, the amount depending upon 

 the relative position of the nuclei in the mitotic cycle when the 

 exposure took place. 



The first visible effect on Planorbis eggs of exposure to X-rays 

 is to stimulate their division. Any particular mitosis which 

 may be in progress at the time of the division is hastened to a 

 very great degree. 2 So far as may be observed in the living 

 egg the process is not different in character from the normal in- 

 direct cell division although the later events in the life of 

 the egg make it certain that something essential in the mechanism 

 has been disturbed but the time which is consumed by a cell 



1 (n) At the time of the exposure of three minutes these eggs showed no sign 

 of division, nor was any change noted at the end of the exposure, although in some 

 cases, what were presumable early maturation spindles, could be seen. One hour 

 and ten minutes later the first cleavage division had taken place, and in forty five 

 minutes more the second furrow had appeared. In one hour from this time the 

 first micromere division had not been completed, but in two and one quarter hours 

 after the second furrow the second micromere quartette had been given off, making 

 the twelve-cell stage. 



Experiments (12) and (13) were exact duplicates of (n). 



2 Compare experiments (8), (10). 



(14) The eggs of this set showed the first trace of the second cleavage furrow at 

 the time of the exposure, which lasted ten minutes. At the end, the eggs were all 

 in the four cell stage. While not all of the cluster had been exactly together at 

 the beginning of the exposure, all apparently finished together. 



(15) At the end of a six minute exposure, eggs in which the second furrow was 

 barely visible at the beginning had now passed into the resting condition. In thirty 

 minutes more the first micromere quartette had been completely given off. Two 

 hours later the eggs were probably in the i6-cell stage, but it was not possible to 

 observe exactly in this case. 



(16) Eggs in the 4-cell stage were exposed three minutes. At the end of the 

 exposure the first micromere quartette had been given off, although in a few cases 

 not quite completely. The four macromeres were as yet spherical, not having 

 flattened against each other; thus they gave the appearance of being almost com- 

 pletely separated from each other. Twenty five minutes later the second quartette 

 had been given off, and at the end of another half hour the first had divided. One 

 hour and thirty minutes later the egg had reached a stage comparable to Holmes' 

 24-cell stage, for which see Fig. 10. 



