CELL DIVISION, MORPHOLOGY, VIABILITY 789 



determination is made. As the treatment becomes lower in intensity 

 and more prolonged in time, the choice of the appropriate time at which 

 the effect is to be determined becomes more and more difficult. Fourth, 

 the cell and its parts undergo a whole series of cyclic changes in their 

 physical and chemical nature during mitosis; treatment extending over a 

 long period of time will increase the chances of a certain highly sensitive 

 stage in the mitotic cycle receiving radiation, while a brief treatment will 

 decrease this possibility. The proportion of a given quantity of radia- 

 tion that a cell receives in a particularly sensitive or insensitive stage, 

 therefore, should not be overlooked in the interpretation of dosage-rate 

 effects. Though recovery processes in the cell usually begin as soon as 

 the first effect of the radiation is produced, damage to the cell during the 

 treatment period — unless the dosage rate is very low — will occur at a 

 faster rate than repair, and the maximum residual effect will be present at 

 the end of the radiation period. From this time on recovery will take 

 place gradually until the capacity of the cell to progress mitotically has 

 been restored to its original state. If, on the other hand, the dosage rate 

 is extremely low, e.g., about 0.8 r/hour in treatment of the grasshopper 

 neuroblast with 7 rays, an equilibrium between radiation damage and 

 tissue repair will be established soon after the start of treatment, the 

 residual effect remaining relatively constant over a long portion of 

 the treatment period (Carlson and Harrington, 1953). Not only is it 

 essential that these factors be taken into account in the interpretation 

 of radiation results, but they may also be utilized in designing experi- 

 ments to test certain hypotheses relating to the effects of radiations on 

 living material. 



The interpretation of the results of studies of the effects of different 

 dosage rates on mitosis in selected cells that were in known stages of 

 mitosis at the time of treatment and that can be observed at desired 

 intervals after treatment, e.g., marine invertebrate eggs and hanging-drop 

 preparations of grasshopper neuroblasts, offers no particular difficulties, 

 if treatment is not so prolonged that deterioration sets in during the 

 course of the experiments. The Arbacia sperm and egg offer, in fact, a 

 unique opportunity for time-intensity studies. As described previously, 

 Henshaw (1932) found that recovery of eggs occurred between treatment 

 and cleavage. Further, Henshaw et at. (1933) found that recovery took 

 place between the beginning and end of irradiation. If Arbacia eggs 

 were exposed to a given dose of X rays administered at different rates, 

 the cleavage delay produced by the lower dosage rate was 20-40 per cent 

 less than a dosage rate eight times as high and therefore given in one- 

 eighth the time. On the other hand, the sperm gives no evidence of 

 recovery before insemination (Miwa et at., 1939a; Henshaw, 1940a). 

 These results would lead us to expect that the Arbacia egg would exhibit a 

 dosage-rate effect, while the sperm would not; and such, indeed, is the 



