2 INTRODUCTION 



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 vigor- 

 ous 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 biologi- 

 cal, they naturally exhibit more or less variability and reflect in 

 each instance something of the conditions under which they have 

 been obtained. It follows therefore that they must not be ex- 

 pected to possess the precision of physical or chemical determi- 

 nations. Nevertheless, so long as the values here presented are 

 not mistaken for absolute standards representing ideal or final 

 determinations, they may be used with advantage. 



Most of the matter presented is taken from researches already 

 published in full, but in a few instances data from work in prog- 

 ress have been included also. In the latter instance the author's 

 name is followed by (MS with date) when it is based on work 

 conducted at The Wistar Institute — while in other instances the 

 laboratory is also named. 



In a few of the published tables — mainly from our own lab- 

 oratory — it has been found necessary to make corrections — so 

 that when the tables here printed do not agree with the origi- 

 nals, it is to be assumed that the changes are due to revision. 



Owing to the absence of tables for the normal animal or to 

 the failure of the authors to express their results in a quantita- 

 tive form, much of the literature which is cited is unaccompanied 

 by any text. Such papers however often contain valuable in- 

 formation on either the Albino or Norway rat and the citation 

 of them serves to indicate the range of the studies in which 

 this animal has been used. 



Extensive reference tables have been computed for the various 

 characters only as these appear under normal conditions, while 

 the modifications which may be experimentally induced in these 

 characters are merely mentioned in the text or presented very 

 briefly. 



