GROWTH OF THE CEREBRAL CORTEX 67 
in the same manner as that for the sagittal section, the dorsal 
and the ventral borderlines of the cortical areg were drawn. 
The dorsal border (d) was determined by the ectal borderline 
of the corpus callosum, which lies under the tip of the cortex 
at the bottom of the fissura sagittalis, and the ventral border 
(v-v’) was drawn perpendicular to the surface at the basal end 
of the cell group which is found under the cortex proper just 
below the region of the fissura rhinalis, latero-basal to the cap- 
Fig. 2 Showing, by shading, the cortical area measured on the frontal sec- 
tion of the albino rat brain. The dorsal border (d) is chosen at the borderline 
of the corpus callosum. The ventral border (v-v’) was drawn perpendicular 
to the surface at the basal end of the cell group found near the fissura rhinalis, 
latero-basal to the capsula externa. The double shaded part, locality VII, 
indicates the area where the cell number and cell size were determined. 
sula externa. This latter border is not so sharply defined, but 
we could not find any better marking point than this cell group. 
The area of the cortex, thus bounded, was then measured and 
recorded. In making the measurement, the area of the cortex 
was at first measured and then the total area of the frontal sec- 
tion,—of one hemicerebrum—excluding the cavity of the lateral 
ventricle and the tractus opticus, was measured. The ratio 
of the cortical area to the total area of the section was then 
computed. Correction of the observed values to those for the 
