FOREBRAIN MORPHOLOGY 415 
therefore exhibit more normal conditions. From the morphologi- 
eal point of view, this part of the forebrain affords a considerably 
greater interest than the caudal one. 
The rostral part of the forebrain is inverted and rostrally 
evaginated.? The bulbi olfactorii are inserted at the rostral 
end of each hemisphere. In the bulbi no bulbar ventricle is 
present. 
The pallial parts are comparatively small, extending along the 
rostral part of the medial border of the forebrain. In the evag- 
inated rostral part the pallium forms the main part of the medial 
wall of the lateral ventricle (fig. 24), but, proceeding toward the 
foramen monroi, the septal pallium gradually retires in the dorsal 
direction (figs. 25 to 28, pall.) and at the foramen monroi it is 
found almost quite dorsal to the ventricle (fig. 27). A medial 
zona limitans is very clearly pronounced, especially in the neigh- 
borhood of the foramen monroi. The lateral expansion of the 
pallium is not great. The lateral limit is marked by a distinct 
zona limitans lateralis (z././.), lying a little below the upper corner 
of the lateral ventricle. Caudal to the foramen monroi the 
pallium is reduced to a dense cell-mass at the upper end of the 
transverse section (fig. 28). Here the pallium is no longer in- 
verted as it passes over into the pallium-free ‘praethalamus’ 
of Edinger. 
The pallial parts of the forebrain are inverted so as to bring the 
two pallia in contact with each other. In contradiction to the 
selachians, the pallia do not join in the mid-line, the holoceph- 
alians thus being more primitive in this respect. In Chimaera the 
two pallia are joined by an ependymal membrane, a tela, which 
in its rostral part forms a short and thin fold (figs. 27, 28, ch.f.) 
hanging down between the two hemispheres. 
This chorioidal fold is of great interest as being the caudal 
continuation of an ependymal fold (figs. 25, 26, ch.f.) belonging to 
the upper wall of the recessus neuroporicus (rec.n.). This recessus 
in the young Chimaera brain is very well marked. From the 
lamina terminalis at the level of the foramen monroi, it extends 
3 My description is based on transverse sections of a young Chimaera brain. 
In older specimens many of the structures described become less conspicuous. 
