Figure 3.

(A) Radial migration of a single neuroblast from the underlying pallial ependyma. (B) Hypothesis of radial formation of the neocortex. Progressive waves of migration arise from a small local region of ependyma, forming a radial, or columnar, collection of cortical neurons of affiliated nature. (C) “Pallio-neuromeric” or “multimeric” origins of the neocortex. Tangential migrations, in conjunction with radial migrations, contribute to the formation of the cortex (4, 5). The cortex has been generally suggested to arise exclusively from the pallial ependyma, by events summarized in A and B. The source of the cells contributing to the tangential migration has been uncertain. Karten (4) postulated that such tangentially migrating neurons were derived from early embryonic neuromeres of the telencephalon (C). This notion was inconsistent with a strict radial migration hypothesis. Recent findings in mammals, however, are now compatible with this older hypothesis by Nauta and Karten (5) but still do not prove it to be true.