POSTNATAL GROWTH IN THE ALBINO RAT 47 



For various skull measurements, Hatai ('08) finds the varia- 

 bility in the adult albino rat slightly less than in man. 



Finally, the question may be raised as to the significance of 

 the relative size of an organ. Why does it vary so greatly in 

 different species and even in individuals of the same species, 

 especially at different stages of development? While a satis- 

 factory answer to this question is impossible in the present state 

 of our knowledge, certain features of interest in this connection 

 may be noted. There are several conditions according to which a 

 priori differences in the relative size of organs might be expected; 

 even though, to a certain extent, a change in the relative size of 

 any part tends, through physiological correlation, to produce a 

 corresponding change in other parts of the body. 



1. The relative size of an organ may vary according to the 

 size of the whole body. On account of mechanical principles, 

 with changes in the ratio of dimensions to surface and mass of 

 the body, and so forth, corresponding changes in the relative 

 size of skin, skeleton, musculature, et cetera, might be expected, 

 as has been pointed out by Welcker and Brandt ('03). 



2. Unequal growth of any (especially of a large) part of the body 

 necessarily involves an inverse change in the relative size of the 

 remaining parts of the body. For example, the relative increase 

 in the musculature during the later periods of growth involves 

 a corresponding decrease in the relative size of other organs. 

 Fluctuations in the relative amounts of intestinal contents, body 

 fat, hair, and so forth, may likewise produce changes in the rela- 

 tive size of the various organs, as has been emphasized by E. 

 Voit ('05). 



3. Differentiation in histological structure may result in more 

 efficient physiological activity, so that a relatively smaller organ 

 may suffice to perform a given function. For example, it is 

 doubtless partly on this account that the embryonic heart and 

 other organs are relatively larger than later. 



4. Changes in functional activity, as is well known, occasion 

 an atrophy or hypertrophy of the corresponding organs, with 

 resultant changes in their relative size. Thus an increase of 

 protein in the diet throws more work upon the liver and kidneys, 



