the rats were divided into groups of two or four, accord- 

 ing to sex. At this age the simple environment of the 

 oage had not beoome monotonous and sleep w&3 not indulged 

 in a greater part of the day. The rats were then excit- 

 able and extremely curious, which conditions were ideal 

 for experimentation. The rats were taken at an age when 

 far less complex activities have been ingrained than the 

 ones they must make in solving the problems. Movements 

 were demanded of the rats which were very different from 

 those usually made in the narrow environment of the cage. 

 A greater part of the v/ork was done in the same room 

 in which the rats were kept, and, as far as practicable, in 

 the morning hours, though sometimes extended into the 

 afternoon. Towards the end of all experimentation the 

 work was conducted chiefly in the afternoon. As soon 

 as food was prepared for the rat s, work was started im- 

 mediately. Ho delay was permitted and the experiment 

 was coxitinued usually without interruption. This was 



