THE ASSOCIATION CENTERS 241 



Sugita (19 1 7-1 8), in a detailed comparative study 

 of the cortex of albino rats and wild Norways, found 

 a greater average thickness of cortex in the wild rats 

 and some other differences. The total number of nerve 

 cells in the cerebral cortex of the two races is about 

 equal, but the average size of the individual cells of 

 the wild rats is notably greater. The mature brain of 

 the Norway rat contains more gray matter than does 

 the brain of a mature albino of like weight. It is high- 

 ly probable that domesticated albino rats actually 

 use their cerebral cortex far less than do wild Nor- 

 ways, and further studies of the minute differences 

 in cortical texture of these two races are very de- 

 sirable. 



In an endeavor to find a simple objective and 

 quantitative indicator of the capacities of animals 

 with different grades of cortical elaboration, Hunter 

 (1913, 1917, igi^a, p. 8) performed his well-known 

 delayed reaction experiment. He trained rats, dogs, 

 raccoons, and young children to make simple associa- 

 tions, as between the appearance of a light with food, 

 and then determined the length of time that may 

 elapse between the presentation of the sensory stimuli 

 and the response before the association is lost. The 

 rats, the dog, and the youngest child (at thirteen 

 months and two and one-half years) were unable to 

 make the association unless they were permitted to 

 consummate the reaction within a few seconds after 

 the application of the initial stimuH, or unless they 

 utilized persistent motor cues, such as the preserva- 



