350 'Journal of Comparative Neurology and Psychology. 



uniform as to size and general appearance. Their weights, how- 

 ever, varied somewhat as can be seen by consuking Table III. 



Owing to the fact that the young cannot very successfully be 

 weaned before 25 days, the early activity could not be definitely 

 ascertained. But from observation of the young previous to this 

 age one perceives that they move very little except when they 

 begin to crawl out of the nest between the fifteenth and the 

 twentieth day. The amount of activity therefore previous to the 

 beginning of this experiment is very little. 



At the age of 25 days these eight rats were placed in eight 

 separate cages. Four of the cages were the revolving ones used 

 in the former experiments. The other four cages were of the 

 ordinary stationary type in which rats are usually reared for 

 laboratory purposes. This arrangement was made in order to 

 see what effect, if any, the voluntary exercise of those in the revolv- 

 ing cages might have on the rate of growth and the longevity. 



TABLE III. 



Weights of rats from the same litter at different ages, showing sex and gain of weight of each. The 

 averages of those in the stationary cages are for the two females only. 



For convenience these rats were numbered from one to eight. 

 Nos. I, 2, 3 and 4 were placed in the revolving cages and 5, 6, 7 



