PART I. LIFE PROCESSES 



HELEN DEAN KIMi 

 MATERIAL AND METHOD 



In the spring of 1919, thirty-six wild Norway rat> 

 brought into The Wistar Institute Animal Colony to be used 

 in starting a strain of captive Grays. These animals, com- 

 prising sixteen males and twenty females, had been trapped 

 in various localities in the outskirts of Philadelphia, and all 

 of them appeared to be vigorous and healthy. The smallest 

 individuals were apparently about three months old at the 

 time they were captured; the largest were adults at least 

 a year old. In spite of the changed conditions of environ- 

 ment and of nutrition to which these rats were subjected, 

 most of them lived for many months and some attained a 

 body weight of over 400 grams. 



Only six of the wild females cast young during the period 

 of their captivity, as far as is known. From these females 

 a total of twenty-one litters, containing 139 young (69 males 

 and 70 females), was obtained. When breeding had ended, 

 parents of litters were killed, measured, and carefully dis- 

 sected ; various organs were weighed, and certain bone meas- 

 urements were taken. The findings were found to airive. in 

 all essential respects, with those for a large series of wild 

 Grays that Doctor Donaldson had examined previously. It 

 is a fact of importance that this experiment was started with 

 animals that were normal in every respect after months of 

 caging, since it indicates that life in captivity for this period 

 did not produce any marked changes in body structure. To 

 what extent body functions were altered could not be de- 

 termined, since practically nothing is known of life pro<- 

 in wild Norways. 



11 



