FOREBRAIN MORPHOLOGY 433 
As to the homology of the nucleus medialis septi in the inverted 
and nucleus olfactorius anterior pars precommissuralis in the 
everted forebrain, some facts seem to support this view: 
1. In Polypterus, where there is a small septum present, the 
nucleus olfactorius anterior (= lobus olfactorius) sends a process 
(pars precommissuralis) into this septal structure. 
2. In Acanthias, where in front the nucleus olfactorius lateralis 
occupies the ventral part of the frontal surface of the hemispheres, 
it sends a cell-lamina into the subpallial septum. This cell- 
lamina is the nucleus medialis septi. 
3. In the embryo of Rana (according to investigations made at 
this institute by Miss G. Sdéderberg) the granule cell-zone in 
front of the hemispheres is continued into the septum by the 
nucleus medialis septi. 
The nucleus taeniae, or entopeduncularis, in teleosts is probably 
a derivation from the nucleus precommissuralis pars superior. 
Certain observations in the ontogeny of Salmo (made at this insti- 
tute by G. Lindén) have shown its derivation from the portion 
of the ventricular wall occupied later by the superior part of the 
precommissural body. In Dipnoi, where a nucleus taeniae is 
also present, it is associated with the tuberculum olfactorium. 
As well in ganoids and teleosts as in Dipnoi, it is penetrated by the 
lateral forebrain bundle. I think there may be no serious objec- 
tion to be made against this homology. 
Hitherto, I have thought, like Johnston, that the nucleus 
taeniae is a homologue of the ‘somatic area’ in selachians. This 
view I do not now find well grounded, as my investigations on 
the selachian brain seem to have shown that the ‘somatic area’ 
in selachians is part of an unusually large nucleus preopticus. 
After the above discussion I venture to make up the following 
scheme of the forebrain homologies in lower vertebrates: 
