HABIT FORxMATION IN ALBINO RATS 3 



rats and mating brother to sister within the same litter, this 

 constituting the closest possible inbreeding. At about thirty 

 days of age the young rats were taken from their mother, and 

 those to be used by the writer were shipped to the Johns Hop- 

 kins University. There were no fatalities in transit and all 

 arrived apparently in good condition. The system of number- 

 ing individual rats for identification was as follows: the first 

 number referred to the generation of inbreeding, the letter 

 ( A or B) to the strain, and the last number was that applied 

 to the individual. For example, 7A90$ may be analyzed as 

 follows: 7th generation inbred, A strain, individual 90, female. 

 Each rat had one or both ears punched or clipped to agree with 

 the individual number, a system in use by Professors Castle 

 and Yerkes at the Harvard laboratories. 



It seemed advisable to secure normal control mating strains 

 from different laboratories in order to avoid any possibility of 

 inbreeding. In addition to our own Hopkins stock there were 

 obtained rats from the Wistar Institute, Columbia University, 

 animal dealers in Baltimore, and from Dr. Herbert M. Evans 

 of the Johns Hopkins Medical School. Care was taken in mat- 

 ing the control series to avoid any approach to inbreeding. As 

 in the case of inbred rats the young were taken from the mother 

 at the age of thirty days. The system of numbering individual 

 control rats for identification was as follows: the first letter, S, 

 signified that it was a standard or normal control rat, letters 

 within parentheses gave the pedigree, and the figures gave the 

 individual number. For example, S(C/EB)70<^ may be analyzed 

 as follows: standard, or normal control series, Columbia father,. 

 Hopkins Medical maternal grandfather, maternal grandmother 

 purchased from a Baltimore dealer, individual 70, male. The 

 same system of ear marking was used as in the case of the 

 inbred rats. 



When taken from the mother males and females were kept 

 in separate cages. According to Watson^ the bearing of young 

 has some effect upon the central nervous system of the white 

 rat; for this reason, and in order to keep conditions constant, 

 neither males nor females used in these experiments were al- 

 lowed to mate. As solitude may be a factor affecting behavior, 



^ Watson : The Effect of the Bearing of Young Upon the Body- Weight and the 

 Weight of the Central Nervous System of the Female White Rat. Journ. of Comp. 

 Neur. and Psych., Vol. XV., No. 6, 1905. 



