GROWTH OF THE CEREBRAL CORTEX 243 
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 and at 2.0 grams is greater than at 1.15 grams_ 
(20 days) by about 45 per cent. 
5. In the first phase the cortex increases its thickness by re- 
ceiving some newly formed cells from the matrix and many 
already formed from the transitional layers and at the same time 
by the general enlargement of the neurons, especialy the cell 
bodies; in the second phase, however, it grows main'y by the 
enlargement of the cell bodies and the growth of the axons and 
dendrites; while during the third phase it thickens only slightly, 
but extends in area as the result of the ingrowing axons and the 
formation of the myelin sheaths and non-nervous structures. 
6. The cortex at the frontal pole increases its thickness very 
rapidly and steadily, continuing to do this even after the end 
of the second phase, while at all the other localities the cortex 
thickens in the same proportion, so that at the end of the second 
phase all the localities reach nearly the full thickness, but main- 
tain their initial relations. The localities heterogeneous in their 
cell lamination show in the course of thickening some deviation 
from the localities which are typical. 
7. The cortex generally attains nearly its full thickness before 
myelination, as shown by the Weigert staining method, occurs 
in it. In the Albino, the cortex has nearly its mature thickness 
at twenty days, just before the young rat is weaned and when 
the brain has attained only a trifle more than half its final 
weight. The growth of the cortex in thickness is therefore 
precocious. 
III. INCREASE IN CORTICAL THICKNESS DURING GROWTH OF THE 
BRAINS OF THE MOUSE AND THE GUINEA-PIG 
Mouse. Isenschmid (711) has made a study of the cortical 
cell lamination in the brain of the mouse and given a map of the 
topographic localization in the hemisphere, which is repro- 
duced here as figure 1. De Vries (712) and Rose (712) have also 
presented a brain map of the mouse according to their studies 
on the cell architecture of the cortex; a map which resembles that 
