362 



GROWTH OF ORGANS IN RELATION TO BODY LENGTH 



fifteen days of age. Examination shows that the longitudinal 

 diameters increase more rapidly than do the transverse — but 

 comparison with the relations found in the Albino shows that the 

 change in the form of the brain with age is less marked in the 

 Norway- — so that the Norway brain always has a rounder appear- 

 ance. Furthermore, inspection of the exposed brain in the Nor- 

 way shows the tips of the frontal lobes to be distinctly more 

 rounded in them than in the Albino. The difference in the 

 form of the cerebrum in the two strains can be brought out 

 by comparing the linear measurements for mature brains of the 

 same weight — table 194. 



TABLE 194 

 Brain diameters of Norway and Albino compared. For the designations see Table 19S 



4. Growth of the cerebral cortex in thickness — wild Norway rat— 

 Sugita {'18a). The method of study here used was the same as that 

 for the Albino and both the planes for the sections and the localities 

 at which the measurements were made correspond to those shown 

 in figures 6, 7 and 8. 



The body weights were all above 17 grams. The measurements 

 on eleven brain weight groups, when corrected for the fresh 

 condition, give the values entered in table 195. ^^ 



The cortex of the Norway rat attains nearly its full thickness 

 when the brain weighs 1.54 grams, corresponding to a body weight 

 of 42 grams and an age of 36 days — chart 66. The average 

 thickness at maturity is about 2.05 mm. thus exceeding that of 

 the Albmo at a like brain weight by some 8 per cent. 



(a) Comparison with Albino. The relations of the cortical 

 thicknesses are quite similar among themselves to those shown 



" The measurement for the frontal section is lacking in the last group — table 

 195 and in chart 66. 



