578 



NAOKI SUGITA 



the ratios between values of any two localities in the same age 

 group are all almost equal. This indicates that the proportional 

 thickness at any region of the cerebral cortex to that of any other 

 region is quite constant throughout the growth of the brain. 

 Moreover it indicates that Ihe thickness of the cortex at ma- 

 turity is directly related to the thickness found at birth and from 

 this we infer that the process of thickening is similar in the 

 several localities but that the amount of material (number of 

 cells) involved in the process differs. 



TABLE 11 



Showing the relation between the initial and final thickness of the cortex at the several 



localities. The order of thickness at maturity is the same as the order of 



thickness at birth 



Thickness of cortex according to sex 



No sex difference in the thickness of the cortex has been de- 

 tected. Among the 125 rats, employed in this study, there are 

 28 females, as indicated in tables 1 and 2. Table 12 shows the 

 comparison of the thickness of the cortex, grouped by sex. Exam- 

 ining this table, it is seen that in Groups III to XIII, on all 

 kinds of sections, there can be detected no difference due to sex, 

 because the differences, greater than the probable error, in the 

 figures within each group can be explained by the differences in 

 the average brain weight. But, Group XVII on the sagittal sec- 

 tions and Groups XIII and XVII on the frontal sections show 



