GROWTH OF THE CEREBRAL CORTEX 529 



line II-II' was drawn so that the distance between the middle- 

 points of the lines /-/' and II-II' is equal to the distance be- 

 tween the middle-points of the lines II-II' and II-II I', when 

 the distances are measured along the cell band of the lam. gang. 

 Locality II represents the central part of the area AA'-BB'. 



Locality IV. The line IV-IV is drawn midway between posi- 

 tions of the localities III and V, determined in the same manner 

 as the position of the line II-II'. This locality represents the 

 central part of the area CC'-DD'. 



The lines /-/' and II-II' represent the thickness of the cortex 

 m the area AA'-BB', the lines IV-IV and V-V the thickness 

 of the cortex in the area CC'-DD', and through the increments 

 in the lengths of these lines according to growth the development 

 of the frontal and occipitals parts of the cortex of the rat brain 

 as they appear in this plane may be determined. 



For convenience of comparison in the final statement I have 

 averaged the values of the above named five measurements and 

 this is designated as the ''average thickness of the cerebral cor- 

 tex in the sagittal section." 



Frontal sections 



In figure 2 the line ff' marks the part of the brain from which 

 the frontal sections were taken (FF' fig. 1). The frontal sections 

 were obtained by cutting the hemisphere in a plane passing 

 through approximately the middle point of the mesial surface, 

 the corpus callosum, the commissura anterior and the chiasma 

 opticum (fig. 1). For the frontal sections the left hemisphere of 

 the same individual, from which the sagittal sections of the right 

 hemisphere had been taken, was used. The technical procedure 

 was similar to that used for the sagittal sections. 



Figure 4 is the general diagram of the frontal section, from the 

 albino rat brain, about thirty days of age, and illustrates the 

 cell-lamination and the positions of the cortical localities which 

 were measured. The cortex of the frontal section shows several 

 areas characterized by the structure of the cell layers. 



At the bottom of the sagittal fissure appears a small area, where 



THE JOUBNAIi OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 3 



