GROWTH OF THE CEREBRAL CORTEX 3l 
has its reason. If the data are treated as follows, which seems 
to me quite a rational treatment, the reason will be disclosed.° 
The brain of the Norway rat at birth weighs usually somewhat 
more than that of the newborn albino rat, and the final brain 
weight in the full grown Norway is ca. 2.5 grams or 25 per cent 
higher than that in the mature albino rat of like age, which 
weighs about 2.0 grams. As already shown by Donaldson and 
ho SSeS + ——— + 
| | | | | | | | | | 5] 
24 }— — 4p +—+ = =! 
} | | | |. | a | | Nix *4 
22 r+ + | tL {___.+ t =: 
ie sire = ele wae 
| | | jee |e | i = r 
| | aie } Aaa ene ~™ = 4 —S at S 
a ee eae aS ee es | ee 
| | ae | 
46+ - = t 
| | 4 4 
a4) T | +— 
| 
42 + a_i IL = jt 
| | 
40 | 
| | | | 
| | | | 
0.8 | | t 
06 + ——t 2 See 
| | | | 
} | | | | | 
04 |— mt + ee a 
| | 
a2) Ss ——= Saale bt rs 1 fact = 
40 if 12 13 14 45 16 17 18 49 20 21 22 23 gre 
Chart 8 Giving the corrected thickness of the cortex in the sagittal, frontal 
and horizontal sections and the general average thickness for each brain weight 
group. Based on table 7. Norwayrat. -—-°-—-—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 ° eA General average thickness of the cortex 
of three kinds of sections. Corrected. 
Hatai (’11), the span of life is probably the same in both the 
forms, extending to about three years. So, if throughout this 
span of life the developmental course of the brains was quite 
similar for both forms, the brains which have like weights would 
not represent the same stage of the development, but on the con- 
trary, a brain of the Norway rat would be under these conditions, 
in a younger stage. 
Table 9 gives the percentage of water in the brains of the Nor- 
way and of the albino rats. The comparison of the data of the 
