PROLOGUE 



In order to get a background for studies on the growth of 

 the nervous system of the rat, the available information on the 

 growth of this animal was brought together and published in 1915, 

 the first edition of this memoir. The time has now come for 

 a revision and a word is in place concerning the changes which 

 appear in this second edition, the significance to be attached to 

 the method of presentation and the manner in which the data 

 and tables can be used. 



Touching the revision it suffices to say that the usual methods 

 of efimination, correction and condensation have been followed, 

 but nowhere are the changes made of a fundamental character. 



Since 1915 our bibliography of the rat has increased by some 

 1300 titles, and these new contributions have furnished a con- 

 siderable mass of information which has been incorporated. 



Despite the fact that the aim has been to use only the best 

 data touching any given point, and not to record the historical 

 growth of knowledge concerning it, this second edition is 

 inevitably larger than the first — a result to be deplored but 

 apparently unavoidable. 



In citing the new results it has been necessary to limit the 

 entries almost entirely to fundamental data — to do more would 

 have been to attempt an encyclopaedia. 



When all is said, the question remains as to the significance 

 and utility of the data and tables as here given. Appreciating 

 that domestication has modifying effects, tables have been 

 made separately for both the wild Norway and the domesticated 

 Albino. Even as they stand these show differences between 

 the two strains and these differences are regarded as due for 

 the most part to the domestication of the Albino. 



The pied strains, which are sometimes used for laboratory 

 purposes, and which are also domesticated, seem to be very 

 similar to the Albino. At the same time albinism is also a 



