10 



been fonnd that white rats are easier to breed, handle, and 

 care for in large numbers than any other anall mammal. For 

 reasona wliich will appear later, we adopted tlie circular maze 

 as our problem since it is generally conceded that the rat is 

 pre-eminent among animals in his ability to tl:read a labyrinth, 

 while his satisfactoriness as a subject for experimental work 

 is attested by the number of experimenters who have employed 

 him in various capacities. 



The rats were bred in our own laboratory as needed, in- 

 breeding being carefully avoided and all possible care being 

 taken to maintain uniformity of breeding conditions. All of 

 the rats were weaned at from eighteen to twenty-three days,* 

 and the sexes were separated at tliirty-five to forty days and 

 kej t seiarate thereafter. The livin,?- cages were protected 

 fror. mice and gray rats by screened compartments cons tri;c ted 

 of pine and 1/4 inch wire mesh. Every two weeks the cages 

 were thoroughly cleaned, the shelves washed with a disinfect- 

 ing solution, and the rats dipped in a one j er cent solution 

 of "kreso" to prevent the rise and spread of vermin. The 

 animals were carefully watched and treated immediately upon 

 the appearance of jarasite^;, so that they were kept continual- 

 ly in a healthy condition. The diet consisted of milk-soaked 

 bread given every day, and a mixture of cracked corn and sun- 

 flower seed every other day. They seemed to thrive on this 

 somewhat restricted diet, so that no additions were made to it 



* :io bed effects were noticed from this early weaning and the 

 rata were found to be extremely active as early as the 16th 

 day. See Slonaker, op. cit. p. SCO. 



