GROWTH OF THE CEREBRAL CORTEX 



587 



The second phase extends from the tenth day to the twen- 

 tieth day after birth. During this phase the cortex receives but 

 few new cells, but the thickness of the cortex increases chiefly by 

 enlargement of cells already present and the developmental 

 lengthening of axons, the rate of the increase in cortical thickness 

 being just about equal to the increasing rate of brain size in one 



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Chart 12 Showing the absolute weights of the dry substance (in grams) in the 

 brain of the albino rat according to the brain weight (in grams). Based on the 

 data given in table 74 in "The Rat" (Donaldson, '15). X, the middle of the sec- 

 ond phase; XX, the early part of the third phase. 



diameter. During this phase, the cortex becomes provided with 

 nearly all of the cellular elements necessary to it at maturity. 

 By the end of this phase, the cortex has attained almost its full 

 thickness, the brain weight, however, being only a little more 

 than one-half that of the mature brain. Myelination has been in 

 progress since the first few days of this phase, but in the cortex 

 the amount of myelination is still very slight. 



