The forebrain of the Pacific hagfish: a cladistic reconstruction of the ancestral craniate forebrain.

The forebrain of the Pacific hagfish is described with regard to its morphology, cytoarchitecture, and secondary olfactory projections. The forebrain ventricular system is greatly reduced in adult hagfishes, although vestiges of ventricular structures can still be recognized. In order to clarify topographical relationships within the forebrain, we provide a three-dimensional reconstruction of the ventricular system, including the vestigial portions. Topography and embryology lead us to conclude that the 'primordium hippocampi' of previous authors is a diencephalic structure. For topographical and hodological reasons, we interpret the 'area basalis' of previous authors to be part of the preoptic region, and we identify a part of the so-called 'nucleus olfactorius anterior' as the homologue of the striatum. The laminated pallium is dominated by secondary olfactory projections and shows a high degree of regional cytoarchitectural specialization, as does the entire forebrain. In all, 42 cell groups are identified in the forebrain of hagfishes (compared to only about 25 in lampreys, for example). This surprisingly high degree of cytoarchitectural complexity prompted us to re-examine the phylogenetic history of craniate brains with this complexity in mind. In this paper we use cladistic methodology to reconstruct a morphotype, and we conclude that the forebrains of the earliest craniates may have been more complex than previously believed. This reconstruction includes hypotheses regarding the general morphology, secondary olfactory system, and visual system, as well as the relative sizes of major divisions of the forebrain in the earliest craniates.