PREFACE TO THE FIRST EDITION 



For a number of studies on the growth of the mammalian 

 nervous system made bj^ my colleagues and myself we have used 

 the albino rat. In the course of the work we frequently felt 

 the need of referring to other physical characters of the rat to 

 which the nervous system might be related. This led us to col- 

 lect such data as were already in the literature and also led us 

 to make further investigations. The facts gathered in this way 

 have proved useful to us and are here presented in the hope 

 that they will be useful to others also. 



The plan of the presentation is simple. An introduction treats 

 of the rat as a laboratory animal, indicates the methods of 

 gathering the data, and also gives examples of our use of the 

 tables. This is followed by an outline of the classification of 

 the common rats and by a brief statement of the history of 

 the rat since it arrived in western Europe. 



The rest of the book falls into two parts. The first part deals 

 with the domesticated albino rat — concerning which we have 

 the larger amount of information. 



The second part deals in a similar way with the wild Norway 

 rat — the form from which the Albino has been derived. In 

 connection with each part the several reference tables and the 

 formulas employed for them and for the corresponding graphs, 

 are given, and at the end of the book a list of papers on the rat 

 is added. 



In the two parts which form the body of the book the purpose 

 is to present for the rat under normal conditions the funda- 

 mental observations — giving data and conclusions only. It is 

 hardly necessary to add that in most directions our information 

 is fragmentary. 



For all the formulas which apply to the data coming from the 

 laboratories of The Wistar Institute, I take pleasure in thanking 

 my colleague, Dr. S. Hatai. 



