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Compared to land vertebrates and the other fishes, the basal ray-finned fishes and teleosts have morphologically unusual forebrains. The telencephalic pallium is everted, and its diencephalic inputs arise largely – not from what is clearly the dorsal thalamus but rather – from an enigmatic group of migrated nuclei of the basal diencephalon. The subpallia exhibit much less variation than the pallia. The polypteriforms have a long, thin pallial sheet that can be divided into four zones which recent data suggest could correspond to the ventral, lateral, dorsal, and medial pallia of tetrapods, with some caveats. In sturgeons and gars, the pallium is thicker and divisible into three or four zones. The data available for sturgeons and gars are not sufficient for the formulation of more than a barely working hypothesis, although a review of the existing literature has illuminated some basic unresolved issues in these taxa. The voluminous literature on the pallium of teleosts is briefly summarized. The thickened pallium is divided into four zones which are compared with those of the polypteriforms and those of the tetrapods. The topographical position of the primary olfactory target in the pallium is lateral, which is unexpected in an everted pallium. Several recent hypotheses have sought to explain this organization: partial eversion, caudolateral eversion and displacement, simple eversion with changed olfactory connections, and not quite so simple an eversion with preserved topology. These hypotheses are evaluated and some persisting problems are enumerated.

Cytoarchitectural analysis of the octavolateralis area of the goldfish, Carassius auratus, reveals that as in other teleosts, five first-order octaval nuclei are present: the anterior octaval, magnocellular, descending, tangential, and posterior octaval nuclei. The descending nucleus appears to be anatomically specialized relative to that of the halecomorph Amia calva and many teleosts in that a large dorsomedial subpopulation of the nucleus lies medial to nucleus medialis, a first-order lateral line nucleus. In addition to this dorsomedial zone, the descending nucleus is made up of an intermediate and a ventral zone.Application of horseradish peroxidase (HRP) to individual inner ear endorgans reveals that the distribution of these afferents to the octaval nuclei is generally similar to that in another otophysan, Ictalurus punctatus [McCormick and Braford, 1993]. Nucleus magnocellularis receives a diffuse projection from all of the endorgans. The semicircular canals project heavily to the nucleus tangentialis, the entire ventral zone and portions of the intermediate zone of the descending nucleus, the ventral portion of the caudal anterior nucleus, and the bulk of the rostral anterior nucleus. The macula neglecta projects to the intermediate zone of the descending nucleus and to ventral locations within the dorsal half of the caudal anterior nucleus. The otolithic endorgans - the saccule, lagena, and utricle - project, in an overlapping manner, to the dorsal half of the caudal anterior nucleus and minimally to the rostral anterior nucleus. The inputs of the otolithic endorgans to the intermediate zone of the descending nucleus are more segregated, though a given region is sometimes supplied by more than one endorgan. The projections of the saccule tend to be concentrated more medially than those of the other two endorgans. The dorsomedial zone of the descending nucleus receives the majority of its primary input from the saccule, and a much smaller input from the lagena, over most of its rostrocaudal extent. At caudal-most levels of the dorsomedial zone, afferents from the three otolithic endorgans overlap.

The morphological organization of the monoamine-containing neurons in the brain of the sunfish (Lepomis gibbosus) was studied by means of the Falck-Hillarp histofluorescence method. No attempt was made to distinguish between norepinephrine and dopamine, both primary catecholamines (CA) yielding a similar yellow-green fluorescence after paraformaldehyde treatment. In the brain stem of this teleost fish, three groups of CA-containing neuronal somata have been found. First, there is a small collection of CA perikarya located just caudal to the obex of the fourth ventricle. The neurons of this medullo-spinal group give rise to numerous CA fibers many of which ascend within the central portion of the medulla. Intermingled with these CA fibers are some CA cells that constitute the cendral medullary group. The CA perikarya of this group are scattered between the levels of cranial nerves X and VIII. The tegmentum of the isthmus also contains a small group of very closely packed CA neurons. The large-sized CA cells of the isthmal group are located dorsolateral to the medial longitudinal fasciculus, partly within the periventricular gray. High densities of CA varicosities were also disclosed in various brain stem structures such as the optic tectum, the torus semicircularis and the cerebellar valvula. In addition, numerous serotonin (5-HTI-type neuronal somata were found in the raphe region of the brain stem, particularly a t caudal mesencephalic, isthmal and rostra1 medullary levels.A large number of CA cell bodies were visualized in the sunfish hypothalamus. Most of them form two populations of small, round cells that are located along and partly within the ependymal walls of the posterior and lateral recesses of the third ventricle. These bipolar cells possess one short club-like process protruding into the ventricle and their thin ependymofugal processes contribute to the CA innervation of numerous hypothalamic regions. Large CA neurons apparently without direct CSF contact also occur in the area of nucleus posterior tuberis, a t the level of the mesodiencephalic junction. Although the hypothalamic inferior lobes are devoid of CA cell bodies they are heavily innervated by CA axons.The sunfish telencephalon also receives a strikingly massive and complex monoaminergic innervation. Numerous CA fibers which are first observed a t the level of the preoptic area, ascend through the central zone of the telencephalon and arborize profusely particularly within the medial zone of area dorsalis telencephali. Other CA fibers, as well a s abundant fine 5-HT varicosities were found in the lateral zone of area dorsalis. Although the exact origin of the telencephalic CA afferents in Lepomis is not known, part of it may arise from the isthmal CA cell group which appears similar to the locus coeruleus of reptiles, birds and mammals. ' Reprint requests should be sent t o Dr. A, Parent,

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