30 NAOKI SUGITA 
If, during the growing period, a comparison of cortical thick- 
ness in brains of like weight is made, the result is somewhat puz- 
zling, as seen in chart 9, which gives the thickness of the cortices 
of the Norway and the albino rats in brains of the same weight. 
‘ 
38 T = = = = ] 
x6 | a = t— | 
| | | | | 
34 t - = VN | 
ees hy Ne TY 
at | | 
ae SESS EERE 
| | | Seine | Al 
a! | | / 
30 si ai i ial { 
| mel | | 
28 + - - + t se 1S 
| | 
26) AE — ed —— 
| ee | | 
24 - + 7 | 
22 | i 
| 
2.0} ; 
al 
48 t 
16 1 + 
44 } ; — 
12|- | d: 
| | 
40} } 
i 
| | | | 
oe T ails flea 
| | | | | 
4 7 | 4 | a | | 
04} =! | | ! Hel =! ———t 
i esata La L 
| 
ala aa Lea 
PLC ne Yee i De fy a T-  S  2 
Chart 7 Giving the average thickness of the cortex for each brain weight 
group at localities IX, X, XI, XII and XIII in the horizontal section and the 
average thickness at five localities in each brain weight group in horizontal sec- 
tion. Basedon table 6. (above the heavy line) Cortical thickness at locality 
IX. Corrected. — -— :— (above the heavy line) Cortical thickness at locality 
X. Corrected. ---------- Cortical thickness at locality XI. Corrected. 
-—-—- (below the heavy line) Cortical thickness at locality XII. Corrected. 
(below the heavy line) Cortical thickness at locality XIII. Corrected. 
*H Average thickness of the horizontal section for each brain weight group. 
Generally the cortical thickness of the Norway rat, whose brain 
weighs more than 1.3 grams, is clearly higher than that of the 
albino rat of like brain weight, while in brains weighing less than 
1.2 grams the relation is reversed. This seems surprising, but 
