428 NILS HOLMGREN 
Acipenser, Scaphirhynchus, and Polyodon the zona is only to be 
seen in the foremost part of the forebrain. There is, however, no 
difficulty in making out the limit between the pallial and sub- 
pallial parts, owing to very pronounced structural differences 
between these parts, the pallial neuropil being denser than the 
subpallial. 
A suleus limitans pallii lateralis generally is not present in 
ganoids and teleosts, except in Amia, where Johnston has found 
such a sulcus, and in Acipenser, where it is present in the fore- 
most part of the forebrain. 
The subpallial parts in ganoids in principle are built up as in 
Polypterus. In Lepidosteus, Amia, and teleosts a very well- 
defined nucleus olfactorius lateralis below the zona limitans is 
present. In Lepidosteus (and Amia) this nucelus is very inter- 
esting, being formed by cells grouped together in islets or clusters, 
very well differentiated in the preparations from all other cells in 
the subpallial parts. Thus, it is here possible to make out the 
limits of this nucleus more fully than in other fishes. The nucleus 
olfactorius lateralis is composed of a lateral part, forming a cell- 
lamina in the lateral portion of the brain-wall (fig. 39, n.olf.l.). 
From this lamina a process extends medially to join the ventricu- 
lar ependymal layer just below the zona limitans, dorsal to the 
precommissural body (p.p.s.).. Through this process the ventricu- 
lar situation of the nucleus olfactorius lateralis is clearly 
demonstrated in Leipidosteus. Johnston has shown that the 
nucleus olfactorius lateralis in Amia is built up almost in the same 
way. The opening of the bulbar ventricle is caudally continued 
by a groove of different shape and length in different fishes. In 
Acipenser this groove is not very pronounced, forming a shallow 
concavity in the foremost part of the forebrain. In Lepidosteus 
the groove is very broad, rostrally forming a semicircular excava- 
tion (fig. 39, s.l.l.) in the ventricular wall. Caudally, near the 
commissural bed, the excavation becomes shallower and disap- 
pears. The ependymal layer, covering the excavation, is thick. 
In Amia (Johnston) the suleus is a narrow ependymal groove. 
In Osmerus (teleost) the rostral part is a relatively broad excava- 
tion, as in Lepidosteus. Caudally this excavation becomes 
