FOREBRAIN MORPHOLOGY 425 
The superior part of the precommissural body in Polypterus is 
very strongly developed, forming a great cell-mass extending 
longitudinally from the caudal margin of the bulbus olfactorius 
to the commissural bed. Transversely it occupies the space 
between the sulcus limitans pallii and the lower vertical part of 
the ventricle. The sulcus limitans externus thus passesalong the 
middle of the pars superior. Laterally, the rostral portion of the 
pars superior extends toward the lateral surface of the brain. 
The caudal part is more ventricular, with fewer cells. 
In the pars superior a subdivision is indicated by the cell forma- 
tion at the ventricular lumen. Above it has been mentioned that 
there is a cell-free space present just under the pallium in the 
rostral end of the brain. More caudally this space is enclosed 
between the sulcus limitans pallii and the sulcus limitans externus. 
At this point rows of small deeply staining cells make their appear- 
ance and soon occupy the upper half of the superior part. This 
represents the nucleus olfactorius lateralis. Under this region the 
cells are more scattered, paler. 
At the rostral end of the brain the dorsal subdivision of the pars 
superior is well separated from the pars inferior, more caudal the 
two nuclei join more intimately. The pars inferior, however, is 
always distinguishable, occupying its position at the wall of the 
medial ventricle. 
The precommissural body forms the commissural bed. ‘This 
part of the nucleus is the pars commissuralis. 
Behind the commissura anterior complex the nucleus preopticus 
begins, dorsally limited by the sulcus limitans pallii lateralis and 
the pallium. 
In all parts of the precommissural body the main part of cells 
are more or less densely grouped along the ventricular wall. In 
the outer portions, however, there are scattered cells present. 
These scattered cells are arranged ventrolaterally from the com- 
missural bed in a diffuse layer through which passes a lateral 
pallial tract on its way to the commissure. In the caudal pro- 
longation of this layer lies a great nucleus entopeduncularis or 
taeniae (Johnston, fig. 41, n.t.), penetrated by the lateral forebrain 
bundle. 
