532 NAOKI SUGITA 



Figure 4 shows also the three locaUties, at which the thickness 

 of the cortex has been measured. 



Locahty VI. The line VI~VI' starts from the tip of the dorso- 

 mesial curve of the pallium perpendicular to the surface and runs 

 parallel to the cell radiation. This line represents the cortical 

 thickness in the area HH'-KK' , where the laminae pyr. et gran, 

 int. are so thin that the sum of the both layers does not amount 

 to one-seventh of the total thickness of the entire cortex. The 

 lam. gang, is somewhat thicker than the lam. mult, and the 

 latter becomes a single layer just at this point. 



Locahty VII. The line VII-VII' has been drawn at the 

 middle of the area LL'-MM'. In this area the laminae pyr. et 

 gran. int. have increased in thickness, so as to amount to about 

 one-third of the total thickness of the cortex, while the lam. gang, 

 has undergone a corresponding diminution. 



Locahty VIII. The line VIII-VIII' is measured at the 

 bottom of the fissura rhinalis. 



The thickness of the cortex in the frontal section is greatest at 

 Vll-Vir, and least at VIII-VIII', the level of the fissura rhi- 

 nalis, while the thickness at VI-VF is intermediate. 



For convenience of comparison I have averaged the values of 

 the three thicknesses and named this the "average thickness 9! 

 the cerebral cortex in the frontal section." 



Horizontal sections 



In figure 4 the line hh' indicates the level from which the hori- 

 zontal sections were taken (HH' fig. 5). The horizontal sections 

 were made by cutting through the entire brain in a plane ap- 

 proximately parallel to the basal surface of the brain and pass- 

 ing through the frontal poles of both hemispheres and the points 

 at which the occipital poles and the paraflocculi touch (fig. 5) . In 

 the young rat brain, this section is tangent to the dorsal surface 

 of the bulbus olfactorius. In adults, however, it cuts somewhat 

 obliquely through the bulbs, because, owing to the rapid develop- 

 ment of the cerebellum in the early days of life, the paraflocculi, 

 used as the marking points, extend dorsad, causing an upward 



