Topography and topology of the teleost telencephalon: a paradox resolved.

Analysis of vasculature in the telencephalic pallium of a teleost allows the considerable depth of the sulcus externus, which lies at the lateral extent of the ependymal attachment, to be appreciated. The depth of this sulcus is compelling evidence for a simple eversion process (an outfolding of the pallial wall of each hemisphere) during telencephalic development in all ray-finned fishes that is not complicated in teleosts by secondary migration of pallial cell groups. A simple eversion process is known to occur in some ray-finned fishes with relatively simple telencephalic cytoarchitecture but has been disputed in teleosts based on the pattern of olfactory tract projections. A resolution to the conflicting hypotheses of pallial relationships across ray-finned fishes and in comparison with other craniate radiations is presented here, based on a re-examination of hodological and histochemical data mandated by this sulcal anatomy.