576 NAOKI SUGITA 



only 2.6 times. But, after this phase locahty VI appears even 

 to decrease sHghtly, while the locahty VII is still increasing. 



Table 10 gives the rapidity of the gain in the cortical thickness 

 at each locality and for each of the three phases, according to 

 the data in tables 6, 7 and 8. The column designated as ''gain 

 during the first phase" shows the percentage value of the thick- 

 ness at the end of this phase as compared with the thickness at 

 birth. The column designated "gain during the second phase" 

 shows corresponding value for the second phase as compared 

 with the thickness at the beginning of this phase. Similar in 

 arrangement is the column giving the gain during the third 

 phase. This table 10 shows very clearly the relative rapidity of 

 growth at every locality and supports the interpretations pre- 

 sented above. 



Speaking generally, the entire cortex, as represented by the 

 thirteen locahties from different parts of brain, shows very rapid 

 development during the first phase (see general average in table 

 10), especially at the frontal pole and along the mid-dorsal as- 

 pect of the frontal section (localities VI and VII). The parieto- 

 frontal parts (represented by locahties II and X) are somewhat 

 slow in development compared with those on either side of them. 



Through the second phase, the parts showing the most rapid 

 development are also the frontal pole (represented by the lo- 

 cahties I and IX) and the parieto-occipital parts (Vepresented by 

 the locahties IV and V), which were slow in starting, and the 

 slowest growth is by the cortex which appears in the frontal sec- 

 tion (represented by the localities VI, VII, and VIII). The 

 growth at the locality XIII is also slow. This latter region has 

 an heterogeneous structure of cortex, displaying a special type of 

 cell-lamination, as shown in figure 6, and this may be associated 

 with the retardation in its development in thickness. Re- 

 markable is the fact that all the localities measured on the 

 frontal sections, namely, the localities VI, VII and VIII, show 

 relatively slow development during the second phase, having 

 already attained nearly their full thickness at the end of the first 

 phase. This is equivalent to saying that they are precocious. 



