444 J. B. JOHNSTON 
Fig. 19 Section in front of the commissures and behind the optic chiasma. 
The lentiform nucleus is large; the crus occupies the clear space below it and 
sends fascicles up through it (mternal capsule). The nucleus of the diagonal 
band forms a thin cell layer between the fiber layer of that band and the medial 
forebrain bundle. The caudate is small. The upper end of the nucleus of the 
diagonal band appears here in the gyrus subeallosus adjacent to the hippocampal 
primordium and the recessus superior. Note the distinct large- and small-celled 
portions of the pyriform lobe. Also the general increase in thickness and more 
open formation of the cell layer of the dorsal pallium. 
Fig. 20 Section through the optic chiasma. The ventricular ridge grows 
smaller and the definite pallial thickening begins. The hippocampus proper 
grows narrow. The hippocampal primordium is only apparently fused with the 
parolfactory nuclei, as explained in the previous paper. The diagonal band 
and its nucleus move to the angle between the basal and medial walls. The sul- 
cus endorhinalis separates the small-celled from the large-celled part of the 
_pyriform lobe. The lentiform nucleus is quite superficial in this and several 
following figures. The clear zones above and below the core-nucleus of the ven- 
tricular ridge and between that and the pyriform lobe are the spaces through 
which internal capsule fibers run to the ridge and the general pallium. 
Fig. 21 Section at the level of the gyrus subcallosus. The changes noted in 
the last section have been carried farther. The lentiform nucleus shows rows of 
cells due to the arrangement of bundles of internal capsule. In this and figure 22 
the diagonal band nucleus merges with the superficial layer of the anterior per- 
forate space and tuberculum olfactorium. 
Fig. 22 Section through the anterior end of the dorsal ventricular ridge. 
The core-nucleus is here divided into two areas by: pressure from the pallial 
thickening. This thickening is the most conspicuous feature of the section. 
Note the development of a layer of peripheral cells in figures 20, 21 and 22. Com- 
pare figure 40. The caudate begins to enlarge and the lentiform to decrease. 
