GROWTH OF THE CEREBRAL CORTEX 273 
ties in the cerebrum are quite the same in the mouse and rabbit 
as in the rat. The development of the cortical thickness has 
proved to be similar in the mouse and guinea-pig: it attains 
nearly its full value at the weaning time of the animal. 
3. The statement that the cortical thickness diminishes from 
the frontal to the occipital pole and from the dorsal to the ven- 
tral aspect probably holds true throughout mammals, including 
man. 
4. The results given by different authors for the cortical 
thickness of human brain (averages or for each locality) are by no 
means in accord. Even for the same locality there are wide 
deviations. The best data indicate that the average cortical 
thickness of the adult human brain is about 3 mm. 
5. The mode of increase in cortical thickness in man according 
to age appears to be similar to that in the albino rat, if the brains 
are compared at equivalent ages. The developmental stage of 
the brain of a new-born child corresponds to that of an albino 
rat of five days of age, and throughout the postnatal life the rela- 
tive growth rate of the rat and man areas 30 to 1. The span of 
life 30 for man corresponds to 1 for the rat and the equivalent 
ages are represented by like fractions of the span of life. The 
human cortex probably attains nearly its full thickness at fifteen 
months, equivalent to twenty days of rat age. 
6. The relative cortical volumes of the albino and the Norway 
rat brains, computed formerly by me (Sugita, ’18 b), appear to be 
proportional to the surface areas of the entire bodies at the like 
age. This relation may be generally applicable within a given 
order of mammalia. The cortical thickness or the brain weight 
is in general only loosely correlated with the body weight or size 
of the animal. 
7. The cortical nerve cells in the cerebrum and in the cere- 
bellum of the albino rat are precocious in their growth, attaining 
almost the full size at twenty days, the weaning time. The 
maturation of the intracellular structures probably continues 
after the size is apparently completed. This process is shown 
also in the mouse. 
8. The size of the Betz giant cells in the adult human cortex 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 29, No. 3 
