432 NILS HOLMGREN 
In the subpallial parts the nucleus olfactorius lateralis in all 
brains, everted as well as inverted, occupies the same position 
in the lateral part of the brain-wall at the sulcus limitans externus, 
just ventral to the zona limitans, and the conditions especially 
in Lepidosteus make the homology of that nucleus very probable. 
Also the fact that a part of the lateral olfactory tract splits up in 
this nucleus both in inverted and everted brains is of the nature to 
settle the homology. 
In the inverted brain the tuberculum olfactorium is formed 
from the neuroblastie layer ventral to the nucleus olfactorius 
lateralis or the sulcus limitans lateralis (externus). In the everted 
forebrain the superior part of the nucleus precommissuralis oc- 
cupies the corresponding part of the ventral wall, and therefore 
must be homologous with the tubereculum olfactorium. That 
there must be a homologue of the tuberculum olfactorium pres- 
ent in ganoids and teleosts follows from the fact that this part of 
the brain is present in selachians, holocephalians, Dipnoi and tet- 
rapods, and in ganoids and teleosts there is no other part to be 
found with corresponding position. The difference between the 
true tubereculum olfactorium and that of ganoids and teleosts is 
that in the latter groups the cells do not form a cortical layer as 
in the former, but keep their original place at the ventricular 
wall. It is, however, possible that the ventricular position is 
secondary in ganoids and teleosts, as well as in Dipnoi and 
Amphibia! 
Ventral to the corpus precommissurale pars superior in ganoids 
and teleosts comes the pars inferior of the same corpus. This 
nucleus is ventricular and forms the ventral part of the brain-wall 
rostral to the commissural bed. Its morphological position is the 
same as that of the nucleus lateralis septi of the inverted brain, 
since it must be borne in mind that the subpallial septum is the 
medial ventral part of the forebrain ventricle which is brought 
into its septal position through the evagination of the hemispheres. 
Thus the morphological situation of the pars inferior of the pre- 
commissural body is that of the nucleus lateralis septi in the 
inverted forebrain. 
