Telencephalic organization in birds is so unusual that many homologies between avian and mammalian telencephalic areas remain controversial. Particularly contested is the avian "neostriatum," which has historically been homologized to either mammalian striatum, lateral neocortex, or endopiriform claustrum. Because homologies between these adult structures have been so difficult to resolve, we have begun to examine how telencephalic development diverges between birds and other vertebrates. To this end, biotinylated dextran was injected into the lateral telencephalon of chick embryos at 3 d of incubation, and the distribution of labeled cells was examined up to 14 d later. The data show that a definite boundary to cellular migration develops just ventral to the neostriatum between 5 and 8 d of incubation. Labeled polyclones within the neostriatum stretch from the ventricular zone to the brain surface and exhibit an increasingly rostrocaudal orientation as development proceeds. Individual polyclones contribute cells to several of the distinct auditory, visual, somatosensory, and olfactory regions within the neostriatum. A comparative analysis suggests that the avian neostriatum develops from a precursor region that in other vertebrates gives rise to olfactory cortex and, when present, to other components of the piriform lobe, such as the endopiriform claustrum and basolateral amygdala. Conclusions about lateral pallial homologies between birds and mammals remain uncertain, however, primarily because so little is known about the development of the lateral pallium in mammals. This lacuna might be filled by applying to mammals the novel fate-mapping method described in the present paper.

Radial glia are known to have a sparse and uneven distribution in the telencephalon of adult birds. The present study utilizes antibodies against vimentin to reveal a more extensive, and more clearly radial, set of radial glia in the chicken telencephalon during the first half of embryogenesis. This initially extensive radial glial fiber system becomes distorted and reduced between 10 and 14 days of incubation. This reduction coincides with the cytoarchitectural differentiation of the telencephalon into its major adult subdivisions. Because developing neurons tend to migrate along radial glial fibers in both birds and mammals, a topological projection of these major subdivisions onto the embryonic ventricular zone along the radial glial fibers suggests hypotheses about lineage relationships that can be tested by subsequent experimental methods. This analysis suggests that the major components of the avian dorsal ventricular ridge, i.e., the ventral hyperstriatum, the neostriatum with its various subdivisions, part of the archistriatum, and probably also the piriform cortex, all derive from overlapping portions of the lateral pallial ventricular zone. Staining with antibodies against neurofilament suggests that this developmental parcellation of the lateral pallial complex is associated with the development of neuronal fiber systems.

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