580 



NAOKI SUGITA 



The increase in the average thickness of the cortex 



According to charts 9 and 10, the growth curve of the cortical 

 thickness shows many features in its course. In the first phase, 

 previously mentioned, during which the brain weight increases 

 from 0.25 gram to 0.95 gram, the curve rises rapidly smd, 

 steadily. If we assume that, throughout this phase, the specific 

 gravity of the brain substance remains unchanged and the vari- 

 ous parts of the brain grow similarly in all dimensions, then the 

 cerebral cortex should increase in thickness in proportion to the 

 cube root of the brain weight (cf. also table 14 and chart 11). 

 But in fact, the relative thickness of the cortex at the end of 

 this phase is 2.34 or almost exactly equal to the square of the 

 cube root (1.53) of the brain weight (volume). If the fact is 



TABLE 13 



Ginni] the absolute increase in thickness of the cerebral corte.r. during each phase of 

 development, accompanied by the average increase per day during each phase 

 and the ratios of this increase in the three phases. Data from table 10 



recalled that in this first phase the increase of the brain weight 

 according to age has been comparatively rapid, it will be seen 

 that the growth of the cortex in thickness is very rapid indeed. 



In the second phase, during which the brain weight increases 

 from 0.95 gram to 1.15 grams (from the tenth day to the 

 twentieth day after birth), the slope of the curve diminishes 

 markedly, and, in the third phase, after the twentieth day, 

 it runs almost parallel to the base line, showing but a slight gain 

 during further brain growth. 



If the rates of increase of the cortical thickness in the three 

 phases are compared in the terms of absolute increase per day, 

 the following values appear (table 13). 



