590 NAOKI SUGITA 



average daily increase is 62 times and in the second phase 7 times 

 as rapid as in the third phase (table 13). This corresponds well 

 with the rate of mitosis in the cerebrum and cell immigration 

 in each phase. Furthermore if the ratio of the increase of corti- 

 cal thickness is compared with that of the increase of one diam- 

 eter of the brain, the former is much greater than the latter in 

 the first phase, almost equal in the second phase, and much less 

 in the third phase. It appears, therefore, that in the first phase 

 the cortex increases its thickness by receiving newly formed cells 

 from the matrix and at the same time by the enlargement of 

 the cell bodies; in the second phase, however, mainly by the 

 enlargement of the cell bodies and the growth of the axons and 

 dendrites; while in the third phase it almost ceases to thicken, 

 but extends in area as the result of the formation of the myelin 

 sheaths. 



10. On comparing the rapidity of growth in the several locali- 

 ties of the hemisphere, it is easily seen that the cortex at the 

 frontal pole increases its thickness very rapidly and continuously, 

 even after the end of the second phase, while at all the other 

 localities the cortex thickens by similar steps, so that at the end 

 of the second phase all the localities reach nearly the full thick- 

 ness. The localities heterogeneous in their cell-lamination show 

 some deviation in their courses of thickening from the localities 

 which are typical. 



11. If the brain weight is taken as the standard of compari- 

 son, no sex difference is to be detected in the cortical thickness. 



12. We conclude that the cortex generally attains nearly its 

 full thickness before myelination in the cortex, as shown by the 

 Weigert staining method, has begun. 



13. The cortex has nearly its mature thickness at 20 days, 

 just before the young rat is weaned. The growth of the cortex 

 in thickness is therefore precocious. 



14. If the relative growth rates of the rat and man are as 

 30 to 1 (Donaldson 'C8), and the development of the human 

 brain at birth coincides with that of the rat at five days of age, 

 then at about the age of fifteen months the human cortex 

 should have attained nearly its full thickness. 



