GROWTH OF THE CEREBRAL CORTEX 



571 



The increase of the cortical thickness according to age, in- 

 stead of brain weight, is given graphically in chart 10, which 

 was plotted by using the above data, converted by calculations, 

 based on the ''Age-Body weight" and the ''Body weight — Brain 

 weight" formulas given in "The Rat" (Donaldson, '15). This 

 chart shows in a dotted line also the brain weight curve according 

 to age. 



Ql Q2 0.3 0.4 0.5 Q6 07 



0.9 10 11 12 13 14 15 lb 17 18 19 M r 



Chart 9 Giving the corrected thickness of the cerebral cortex in sagittal, 

 frontal and horizontal sections and the general average thickness of the cortex 

 for the three sections combined, on brain weight groups. Based on table 9. 

 • — • — • — S. Average thickness of the cortex in sagittal section. Corrected. 



F. Average thickness of the cortex in frontal section. Corrected. 



H. Average thickness of the cortex in horizontal section. Corrected. 



• "A. General average thickness of the cortex for all three sections. 



Corrected. 



On examining chart 9 (for the cortical thickness on brain 

 weight), I was inclined in the first instance to conclude that the 

 course of the cortical growth in thickness should be divided into 

 three phases; that is, a first phase during which the brain weight 

 increases from 0.25 gram (at birth) to 0.75 gram, and during 

 which the increase in thickness is rapid; a second phase during 

 which the brain weight increases from 0.75 gram to 1.15 grams, 



