THE RAT 



INTRODUCTION 



The Norway rat, Mus norvegicus, is one mammal now easily 

 obtainable both wild and as a domesticated form. This latter 

 is represented by either the Albino or the pied rats so common 

 in our laboratories. 



The Albinos are clean, gentle, easily kept and bred, and not 

 expensive to maintain. They are omnivorous, thriving well on 

 table scraps. The span of life is about three years and breed- 

 ing begins at about three months. Furthermore the species is 

 cosmopolitan. The litters are large and may be had at any 

 season. The young are immature at birth. The domesticated 

 Albino crosses readily with the wild Norway. The rat, both 

 wild and domesticated, takes exercise voluntarily and is sus- 

 ceptible to training. It is also highly resistant to the usual 

 wound-infecting organisms. For a number of lines of study 

 therefore, the rat is peculiarly suitable. 



Through the researches of a host of investigators both in this 

 country and abroad, there has been gathered a considerable 

 body of data applying to the weight and size of the domesticated 

 Albino rat and its parts, as well as some similar data applying 

 to the wild Norway rat, the parent species. It is the body of 

 facts so gathered that it is our purpose to present, as far as 

 possible in tabular form. 



Attention should be called to the fact that the observations 

 presented in the tables have been made mainly on rats in the first 

 year of life and but rarely on those which are older. It follows 

 from this that the data apply to the rat in its most vigorous 

 period and do not give information that can be used for the 

 study of old age. 



Since the quantitative data appearing in the tables are bio- 

 logical, they naturally exhibit more or less variability and reflect 



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