574 NAOKI SUGITA 



2 per cent or less (0.03 to 0.05 mm.). The variations during this 

 period are illustrated by the following records. 



Making the determination of the average deviation from the 

 mean thickness of the cortex for Groups XIII to XVII inclusive 

 (tables 6 and 7), it appears that for the sagittal sections this 

 deviation is ±2.9 per cent and for the frontal sections ±2.7 per 

 cent, while for the equivalent groups (table 8) giving the values 

 for the horizontal sections is also ±2.7 per cent. From these 

 results we conclude that after the period of rapid growth the 

 thickness of the cortex follows closely the brain weight. 



On the basis of chart 10 (the cortical thickness on age) , I have 

 concluded that during the earlier part of the life of the albino 

 rat we may recognize three phases, in the growth of the cortex 

 in thickness, namely; 



1) First phase, from birth to the tenth day. (The brain weight 

 increases during this phase from 0.25 gram to 0.95 gram). 



2) Second phase, from the tenth day to the twentieth day. 

 (The brain weight increases during this phase from 0.95 gram to 

 1.15 grams). 



3) Third phase, from the twenty-first day to the ninetieth 

 day. (The brain weight increases during this period from 1.15 

 grams to 1.80 grams). 



During these phases the various localities, at wljich the thick- 

 ness of the cortex has been measured, grow at different rates. 

 The one which shows the most rapid development in thickness 

 throughout life, is the locality I at the frontal pole (fig. 2). The 

 cortex at locality I attains at the end of the first phase 2.5 times, 

 at the end of the second phase 2.8 times and at the end of the 

 third phase 3.2 times the thickness which it had at birth. At 

 localities II, III, IV, X, XI and XII (figs. 2 and 6), namely those 

 parts of the cortex near the middle of the brain, the development 

 of the cortical thickness is similar. Thus these reach at the 

 end of the first phase on the average 2.3 times, at the end of the 

 second phase on the average 2.5 times and at the end of the 

 third phase on the average 2.6 times the initial thickness at 

 birth. The locality VI, which is situated at the margin of the 

 sagittal fissure, and the locality VII, which represents the parie- 



