38 NAOKI SUGITA 
VIII. SUMMARY 
1. The thickness of the cerebral cortex of the Norway rat has 
been systematically investigated, employing as material 36 males 
and 18 females, all over 17 grams in body weight, and using uni- 
formly the methods which were adopted by me for the investi- 
gation of the cerebral cortex of the albino rat. 
2. The observed data are first given and later are corrected to 
the values for the fresh condition of the material. The corrected 
data are given fully in tables and in charts. ; 
3. The relations of the cortical thicknesses at the several lo- 
calities measured are quite similar among themselves to those 
found in the albino rat. The average thickness of the cortex in 
the adult Norway rat is always higher (1 to 28 per cent) than 
that of the corresponding locality in the adult albino rat. The 
occipital cortex is better developed (thicker) in the Norway rat. 
This is to be associated with the more perfect visual apparatus 
in the Norway rat. 
4. As to the phases of development of the cortical thickness, a 
Norway brain of a given age corresponds to an albino brain, 
which weighs about 16 to 20 per cent less. The Norway brain 
weighing 0.25 to 1.16 grams (Groups N II to N XI) is inits 
first phase of active development which corresponds to an Al- 
bino brain weighing 0.25 to 0.95 grams. The Norway brain 
weighing 1.16 to 1.44 grams (Groups N XI to N XIV) is in its 
second phase of development of the cortex corresponding to the 
albino brain weighing 0.95 to 1.15 grams. " 
5. The cortex of the Norway rat attains nearly its full thick- 
ness at the time when the brain weighs somewhat more than 1.44 
grams, corresponding to the age of twenty days and to a body 
weight of something more, than 36 grams. At this phase probably 
the rapid myelination of the fibers in the cerebral cortex is taking 
place. 
6. The general average thickness of the cortex in the mature 
Norway rat is 2.06 mm., exceeding by about 8 per cent that of 
the albino rat brain of the same weight. 
