INTRODUCTION 



Since 1919 a strain of wild gray rats (Rattus norvegicus) 

 has been maintained at The Wistar Institute for the purpose 

 of determining the changes that occur in these animals when 

 bred in captivity for a number of generations. My colleague, 

 the late Dr. Henry H. Donaldson, was cooperating in this 

 investigation and tracing alterations in the body and organs, 

 while I was studying changes in various life processes. A 

 report on this work, dealing with data for the first ten genera- 

 tions born in captivity, was published some years ago (King 

 and Donaldson, '29). 



At the twenty-fifth generation the detailed study of rats in 

 each succeeding generation was discontinued, and subse- 

 quently only individuals in every fifth generation were under 

 observation. This generation, therefore, ends the period of 

 captivity to be covered by a second report on this strain. 

 Data for life processes in the eleventh to the twenty-fifth 

 generation are given in this paper. Doctor Donaldson's rec- 

 ords for organ changes in these generations will be published 

 later. 



Throughout the course of this investigation the housing 

 conditions, care of animals, and methods of obtaining and 

 recording data were not changed. Although food constituents 

 varied somewhat at times, the rats always received a cooked 

 ration, supplemented twice each week by raw vegetables and 

 fruit. No attempts were ever made to stimulate growth and 

 reproduction by the use of hormone extracts or special diets, 

 nor were any individuals treated with x-rays, radium or other 

 agencies in an endeavor to induce mutations. Thus the strain 

 was maintained constantly under fairly uniform environ- 

 mental and nutritive conditions that furnished adequate food, 

 security from enemies, and protection from extreme tem- 

 peratures. 



AS 



