12
JERALD J. BERNSTEIN
an eversion of the dorsal part of the lateral wall of the telencephalon
concomitant with an expansion of the mass of the telencephalic hemispheres. This eversion is not followed, as in mammals, by evagination
and involution of the hemispheres ( Aronson, 1963; Nieuwenhuys, 1966,
1967b). The eversion of the telencephalon results in the following early
telencephalic characteristics: The lines of attachment of the roof plate
are shifted laterally and ventrally in comparison to their original dorsomedial position; the ependymal cells which only line the ventricles in
mammals are found on the outer surface of the telencephalon of fish and
the main blood vessels that supply the telencephalic parenchyma are
found to enter only to the ventral surface (Nieuwenhuys, 1962a,b;
Segaar, 1965; Segaar and Nieuwenhuys, 1963). These embryonic characteristics result in the location of the highest density of neurons at the
periphery of the telencephalic wall; the ncrve cells appear to be inverted
in comparison to higher forms of animals. The ependymal cells possess
long processes that converge toward the basal surface of the adult fish
forebrain. The vascular, ependymal, and glial patterns persist into adulthood and are extremely characteristic of the eversion process (Nieuwenhuys, 1967b). However, there are many variations on the eversion theme.
In the shark there are isolated islets of neurons and glia (Klatzo, 1967).
In the primitive Polypteriformes, a unique actinopterygian fish, the eversion of the telencephalon has not been accompanied by the thickening
of the telencephalic wall and thus the telenccphalon is saclike in appearance. In contrast, in the goldfish Carassius auratus, the telencephalic
walls are greatly thickened and in dipnoans the thickening is accompanied by slight involution of the telencephalic walls ( Aronson, 1963;
Nieuwenhuys, 1967b).
Therefore, it is difficult to determine if the telencephalon of the fish
has evolved along separate lines or contains areas homologous to other
vertebrate species. For this reason two basically different systems of
nomenclature have been developed for the telencephalon ( Fig. 4). There
are workers who have homologized areas of the telencephalon according
to certain characteristics found in other vertebrates. This represents the
classic approach to comparative neuroanatomy ( Ariens Kappcrs et al.,
1960; Droogleevcr Fortuyn, 1961; Schnitzlein, 1964). In general three
criteria have been used to establish homologies: (1) location and general relation of a zone or mass of parenchyma, ( 2 ) the histology and
cytology of the zone, and ( 3 ) the efferent and afferent connections of
the arca. Using these criteria, it appears that primordia and thus
homologs of various areas of the telencephalon (which are adequately
developed in higher vertebrates) can be homologized in the fish
forebrain.
JERALD J. BERNSTEIN
an eversion of the dorsal part of the lateral wall of the telencephalon
concomitant with an expansion of the mass of the telencephalic hemispheres. This eversion is not followed, as in mammals, by evagination
and involution of the hemispheres ( Aronson, 1963; Nieuwenhuys, 1966,
1967b). The eversion of the telencephalon results in the following early
telencephalic characteristics: The lines of attachment of the roof plate
are shifted laterally and ventrally in comparison to their original dorsomedial position; the ependymal cells which only line the ventricles in
mammals are found on the outer surface of the telencephalon of fish and
the main blood vessels that supply the telencephalic parenchyma are
found to enter only to the ventral surface (Nieuwenhuys, 1962a,b;
Segaar, 1965; Segaar and Nieuwenhuys, 1963). These embryonic characteristics result in the location of the highest density of neurons at the
periphery of the telencephalic wall; the ncrve cells appear to be inverted
in comparison to higher forms of animals. The ependymal cells possess
long processes that converge toward the basal surface of the adult fish
forebrain. The vascular, ependymal, and glial patterns persist into adulthood and are extremely characteristic of the eversion process (Nieuwenhuys, 1967b). However, there are many variations on the eversion theme.
In the shark there are isolated islets of neurons and glia (Klatzo, 1967).
In the primitive Polypteriformes, a unique actinopterygian fish, the eversion of the telencephalon has not been accompanied by the thickening
of the telencephalic wall and thus the telenccphalon is saclike in appearance. In contrast, in the goldfish Carassius auratus, the telencephalic
walls are greatly thickened and in dipnoans the thickening is accompanied by slight involution of the telencephalic walls ( Aronson, 1963;
Nieuwenhuys, 1967b).
Therefore, it is difficult to determine if the telencephalon of the fish
has evolved along separate lines or contains areas homologous to other
vertebrate species. For this reason two basically different systems of
nomenclature have been developed for the telencephalon ( Fig. 4). There
are workers who have homologized areas of the telencephalon according
to certain characteristics found in other vertebrates. This represents the
classic approach to comparative neuroanatomy ( Ariens Kappcrs et al.,
1960; Droogleevcr Fortuyn, 1961; Schnitzlein, 1964). In general three
criteria have been used to establish homologies: (1) location and general relation of a zone or mass of parenchyma, ( 2 ) the histology and
cytology of the zone, and ( 3 ) the efferent and afferent connections of
the arca. Using these criteria, it appears that primordia and thus
homologs of various areas of the telencephalon (which are adequately
developed in higher vertebrates) can be homologized in the fish
forebrain.