Subdivision of the Hemisphere
The embryonic hemispheric vesicle (A) clearly shows the subdivision of the telencephalon into four parts, some of which develop early (phylogenetically old por-tions), while others develop late (phylo-genetically new portions). The four parts are the paleopallium, the striatum, the neopal-lium, and the archipallium.
The hemispheric wall is called the pallium, or brain mantle, because it covers the dien-cephalon and brain stem and envelops them like a mantle.
The paleopallium (blue) (AB1) is the oldest portion of the hemisphere. It forms the floor of the hemisphere and corresponds, with the olfactory bulb (A2) and adjacent paleo-cortex, to the olfactory brain, orrhinencephalon, in the narrower sense. Theneostriatum (deep yellow) (AB3) develops above the paleopallium; it, too, is part of the hemispheric wall, although it does not appear on the outer aspect of the hemisphere.
The largest area is made up by the neopal-lium (light yellow). Its outer aspect, the neo-cortex (AB4), develops very lateand encircles ventrally a transitional area to the paleocortex that lies over the striatum; this is the insula (B14).
The medial hemispheric wall is formed by the archipallium (red) (AB5), an old por-tion of the brain; its cortical band, the archi-cortex, later curls up to form thehippocampus (Ammon‚Äôs horn).
The relationships in the mature brain are determined by the massive expansion of the neocortex, which pushes the paleocortex and the transitional cortex of the insula into the deeper parts of the brain. The archicor-tex becomes displaced caudally and appears on the surface of the corpus callosum only as a thin layer (B5, F10).