C. M. JACKSON 



INTRODUCTION 



The present paper includes statistical data upon certain phases 

 of the growth of the body as a whole and more especially the 

 relative growth and variability of the individual organs in the 

 albino rat. A study of this kind may be of value in two respects. 

 In the first place, it should give a better insight into the nature 

 of growth, a fundamental biological phenomenon worthy of more 

 study for its own sake. In the second place, a more complete 

 knowledge of the process of growth, including the limits of nor- 

 mal variation, should be of value for reference in experunental 

 work of various kinds, for which the rat is often used. 



For both these purposes there is great need of a series of com- 

 plete growth norms, comparable to KeibeFs morphological 'Nor- 

 mentafeln.' A complete growth norm for any given species would 

 involve adequate data upon the prenatal and postnatal growth 

 of the body as a whole, and of its component parts, organs, tis- 

 sues and cells. It would include a determination of the extent 

 of variability due to intrinsic or hereditary factors and to extrin- 

 sic or environmental factors. It is evident that a single fixed or 

 absolute growth norm for a given species does not exist. We 

 can, however, discover the norm and Imiits of variation within a 

 group of animals homogeneous in constitution and in an environ- 

 ment as constant as possible. Furthermore, it is possible to 

 determine to what extent the norm and variability are changed 

 by varying the different factors involved. 



Such a complete growth norm has not as yet been even approx- 

 imately determined for any species, although numerous observa- 

 tions have been made upon different phases of growth in various 

 animals. For the rat, extensive data on the growth of the body 

 and of the central nervous system are available in a series of 

 papers by Donaldson and Hatai. The relative growth of the 

 principal parts and systems of the rat has recently been studied 

 by Jackson and Lowrey ('12); and formulas for the growth of 

 the individual viscera have been determined by Hatai ('13) and 

 are published in the present number of this journal. Further 

 data upon the growth and especially the variability of the body 

 and of the various organs are included in the present paper. 



