GROWTH OF THE CEREBRAL CORTEX 589 



on the cortex of any mammal, which aims to represent the thick- 

 ness of the cortex when fresh. 



3. The data are presented in tables and in charts. The cor- 

 tical cell-lamination of the albino rat is critically reviewed, and 

 the characteristic appearance of the cortex in the newborn is 

 described. 



4. The cortex at the frontal pole of the hemisphere is the thick- 

 est and that at the occipital pole is the thinnest. Speaking in 

 general terms, the cortex diminishes in thickness from the frontal 

 to the occipital pole and from the dorsal to the ventral aspect. 



5. The cerebral cortex of a newborn rat is somewhat less ma- 

 ture than the cortex of man at birth. In the newborn rat, the 

 general average thickness of the cortex is 0.74 mm., correspond- 

 ing to a brain weight of 0.25 gram. 



6. After birth, the general average of the cortical thickness 

 increases very rapidly during the first ten days, thickening to 

 1.73 mm., or more than twice the thickness at birth, while the 

 brain weight increases to 0.95 gram during the same phase. 

 This is designated by me as the first phase of the cortical develop- 

 ment in the albino rat in its postnatal life. 



7. Between the tenth day and twentieth day after birth, 

 the cortical thickness increases more slowly, attaining at twenty 

 days to within 4 per cent of the full thickness of the cortex, namely 

 1.84 mm., or about 2.5 times the thickness at birth, while the 

 brain weight increases to 1.15 grams. This is designated the 

 second phase of the cortical development in the albino rat. 



8. From the twentieth to the ninetieth day, the cortical 

 thickness increases but little on the average, attaining at about 

 90 days the thickness of 1.93 mm. or 2.6 times the thickness at 

 birth, while the brain weight has increased to about 1.80 grams. 

 This is designated the tliird phase of the cortical development 

 in the albino rat. 



After the ninetieth day, there is no significant change in the 

 thickness of the cortex, but the area of the cortex increases as 

 the brain weight rises towards two grams. 



9. On comparing the rate of increase in the cortical thickness 

 during the three phases, we find that during the first phase the 



