424 NILS HOLMGREN 
is relatively thick, with not very densely packed, relatively pale- 
colored cells. The lateral part contains densely packed, dark- 
colored cells and is thicker than the dorsal. Thus these subdivi- 
sions are histologically differentiated. As will be made more than 
probable, they represent cortical layers corresponding to the pyri- 
form (p.c.), the general pallial (g.p.c.) and the hippocampal (h.c.) 
cortices in other vertebrates. 
Subpallial parts. As already mentioned, the bulbi olfactorii 
contain a great bulbar ventricle, opening into the medial fore- 
brain with a foramen monroi-like opening. As this opening is 
bounded ventrally by subpallial parts, dorsally by pallial, it 
might perhaps be compared with the foramen monroi in other 
vertebrates. This would, however, be to commit a great error, 
the dorsal boundary being the morphological lateral part of the 
pallium, the pyriform lobe, and the dorsal boundary of the foramen 
monroi being in other vertebrates the medial part of the pallium, 
the hippocampal lobe. ‘The true foramen monroi thus should lie 
between the subpallial boundary of the opening and the external 
border of the everted pallium. 
The bulbar opening is, as said above, ventrally limited by a 
subpallial part. This at-the foramen forms a small subpallial 
septum (fig. 36, sp.). In this septum enters a process of the 
nucleus olfactorius anterior (p.n.o.a.) or lobus olfactorius. The 
cells in this process stain somewhat darker than those of the 
anterior olfactory nucleus. The process comes from the ven- 
tromedial part of the granule-cell layer (fig. 35, p.n.o.a.) and 
corresponds perfectly to the process of the same nucleus described 
by the present writer in Osmerus. ‘The position of this process in 
the septum of Polypterus makes it probable that it represents a 
septal nucleus, the nucleus medialis septi in other vertebrates, 
this nucleus being many times a backward continuation of the 
nucleus olfactorius anterior or lobus olfactorius. 
As in other fishes, the precommissural body is present in Polyp- 
terus. It is subdivided into a pars precommissuralis superior 
(figs. 37, 38, p.p.s.) and inferior (p.p.t.), a pars commissuralis, 
and a pars postcommissuralis or nucleus preopticus. 
