650 SYLVAN J. KAPLAN 



of trials to achievement of criterion, no other significant differences between 

 the younger groups and control groups were recorded. 



4. In most cases, measures showing significance of differences favored the 

 younger groups over the older ones. Exceptions should be noted: 



(a) The 25 r/2.5 day older group did not differ significantly in errors 

 and trials to criterial achievement from the 10 r/8.5 day, 50 r/8.5 day, 

 50 r/7.5 day, 100 r/8.5 day, 150 r/8.5 day, or 200 r/1.5 day younger 

 groups. Neither did this older group differ from the 50 r/7.5 day, 

 100 r/8.5 day, or 200 r/1.5 day groups in running time. Compared with 

 the 25 r/8.5 day younger group, they were inferior in running time and 

 total errors, but no reliable difference was reflected in the mean number 

 of trials to criterial achievement. 



(b) The older control group was significantly superior to the 10 r/8.5 

 day, 25 r/8.5 day, 50 r/7.5 day, and 50 r/8.5 day younger groups in mean 

 number of trials until the criterion was reached. It was superior in time 

 and error scores compared with the 150 r/8.5 day and the 50 r/7.5 day 

 younger groups. No other significant differences were apparent. 



5. When age and dose level were constant in 400-day-old rats given 50 r, 

 the 6.5 day group had significantly larger mean time scores than the 11.5 

 day group, and no differences between 6.5, 11.5, and 12.5 day groups were 

 found other than those stated before. In 90-day-old rats given 50 r, the 

 7.5 day group was significantly inferior to the 8.5 day group on all measures. 



6. When age was held constant and various dose levels and days of 

 in utero irradiations were compared in 400-day-old rats with 100 r vs 50 r 

 and 25 r, the 100 r/8.5 day animals (group F) did not differ from the 

 groups given 50 r (G,D,E) on any measures, nor from the group given 25 r 

 (B) in mean time scores. This group was inferior to the 25 r/2.5 day older 

 subjects in trials and errors to achievement of criterion. 



In 90-day-old rats with 200 r vs 150 r, 100 r, 50 r, 25 r, and 10 r, 

 group h (200 r/1.5 days) was superior to groups b (10 r/8.5) for time, 

 c (25 r/8.5) for time, d (50 r/7.5) for time and errors, and g (150 r/8.5) 

 for time and errors. It showed no other differences between its control group 

 or other younger groups. In 90-day-old rats with 150 r vs 50 r, no differences 

 were found between group d (50 r/7.5 day) and group g (150 r/8.5 day). 



All animals in all groups failed to meet the criterion of mastery in 50 

 trials (Fig. 2). Only 29, 25, 66 and 50% of the subjects of groups C, D, E, 

 and F, respectively, were able to achieve this criterion. The bar graphs 

 reflect the means for each of the groups for only those subjects which did 

 attain mastery. These data magnify the differences already described. They 

 also reflect the variability of performance in the old animals, particularly 

 those in group C, the 50 r/6.5 day group. 



Three animals in the 50 r/12.5 group showed locomotor difficulty in the 



