FOREBRAIN MORPHOLOGY 429 
narrower and disappears, leaving an ependymal thickening as 
witness to its earlier existence also in the caudal region of the 
forebrain. 
In all fishes the preecommissural body is divided into an upper 
and a lower portion: pars superior and pars inferior. <A 
nucleus olfactorius anterior pars precommissuralis I have found 
in Osmerus in the same position as in Polypterus, but extending 
further backwards. In ganoids I have found no such nucleus, 
owing probably to the preservation of my material. In Polyp- 
terus, as in all ganoids and teleosts, the preoptic nucleus is 
composed of a magnocellular and a parvocellular part. Also a 
pars recessi is present. The recessus preopticus is very broad in 
ganoids, narrower in teleosts. 
The nucleus taeniae (entopenducularis) in ganoids and teleosts, 
as in Polypterus, is penetrated by the lateral forebrain bundle. 
Summary of the forebrain in Polypterus, ganoids, and teleosts 
The forebrain in all fishes'is built up according to a common 
scheme, but Polypterus, in the character of the eversion of the 
pallium and in the broad medial ventricle, occupies a separate 
place. The shape of the pallium should perhaps be taken as 
argument for an idea of a special eversion process, independent of 
that in other fishes. The fact, however, that the subpallial parts 
do not differ from those in other fishes seems to overthrow such a 
theory. Thus, I think that the everted forebrain in Polypterus is 
to be derived from the same source as that in other fishes with 
everted forebrain. Polypterus, thus, in this respect belongs more 
to the actinopterygian stem than to the crossopterygean-dipnoan. 
The scheme of forebrain construction in all fishes with everted 
forebrain thus may be that of the diagram, figure 40. In this 
diagram all nuclei are brought back to the probable position of 
their matrix formations at the ventricular wall. Beginning from 
the dorsal border of the pallium and following the inside of the 
ventricle ventrad, we meet the following structures: 
