GROWTH OF THE CEREBRAL CORTEX 573 



IX. DISCUSSION 



Table 5 and chart 1 show the mean observed values on the 

 slide, while tables 6 to 9 and charts 3 to 9 give in detail the cor- 

 rected values which are assumed to be the actual thickness of the 

 cortex in fresh condition. The corrected data will be taken as 

 the basis for the discussion which follows. 



Brains having almost the same weight rarely show exactly 

 the same cortical thickness, but differ somewhat in this charac- 

 ter. This may be due partly to the technical differences during 

 preparation, but it also means probably some individual varia- 

 tion. Generally speaking, the thickness of the cerebral cortex is 

 well correlated with the brain weight, so that, as a rule, the cor- 

 tex increases its thickness as the brain increases in weight. With 

 the albino rat, the first ten days after birth is a period of rapid 

 growth especially for the central nervous system, so that, at 

 the age of ten days the brain weight has attained nearly four 

 times its weight at birth, growing from 0.25 gram to 0.95 gram, 

 while the body weight has increased only 2.4 times — from 5 

 grams at birth to 12 grams on the tenth day. Accordingly, the 

 cerebral cortex which follows the brain weight also shows a very 

 rapid increase during this period. In this case, as in other cases 

 of rapid growth, considerable individual variations naturally 

 appear. For example, although 'V a' and 'V b' (table 6) are 

 nearly alike in brain weight, the thickness of the cortex differs 

 on the average as much as 0.10 mm. in the sagittal sections. 

 Again, TV d' and TV e' with brain weights almost the same, show 

 a difference of 0.12 mm. in the average thickness of the sagittal 

 sections. These variations amount to ± 5 per cent of the mean 

 value for the cortical thickness. 



Even thirty days after birth, when the phases of the rapid 

 growth of the cortex have already passed, the individual varia- 

 tion in the cortical thickness is by no means low. During the 

 second and third months after birth, the body weight increases 

 from 30 grams to 150 grams or 400 per cent, while the brain 

 weight increases only from 1.30 grams to 1.70 grams or 30 per 

 cent. Through this period, the thickness of the cortex gains but 



