572 



NAOKI SUGITA 



when the rate of cortical increase diminishes; and a third final 

 phase of very slow increase. But this division according to the 

 brain weight is not suitable for comparative studies. If, how- 

 ever, we examine chart 10 (c'ortical thickness on age), it will be 

 readily seen to be more advisable to divide the developmental 

 phases of the cortical growth according to age rather than ac- 

 cording to the brain weight; thus, according to age, the first 



Cortical thickness in 

 Brain weight in gn 



8 10 \% \\ 16 18 20 22 24- 26 28 30 32 34 56 3« 



An, \n days. 



Chart 10 Showing the thickness of the cerebral cortex and the brain weight 

 of the albino rat according to age, up to 38 days. The heavy line gives the mean 

 cortical thickness in millimeters. The dotted line gives the brain weight in 

 grams. 



phase of the rapid increase covers the first ten days after birth 

 (brain weight 0.25 to 0.95 gram), the second phase of slower in- 

 crease covers the following ten days (brain weight 0.95 to 1.15 

 grams) and the third phase of very slow increase in the thick- 

 ness of the cortex lasts from twenty-first day to the ninetieth 

 day and, if necessary, a fourth phase for the remainder of the 

 fife span might be added. 



As is seen in chart 10, the increase of the cortex in thickness 

 according to age follows closely, during the first two phases, 

 the increase of the brain weight according to age. 



