The telencephalon of mammals is characterized by the presence of a hexalaminated structure on its external surface, with specific auditory, visual, somatosensory and motor regions. Due to its seeming unique presence in mammals, it is frequently designated as the neocortex. The evolutionary origins of the so-called neocortex have long puzzled comparative neuroanatomists, in view of the seeming absence of a neocortical-like anlage in nonmammalian amniotes. The resolution of this puzzle requires analysis of both adult and embryonic brains. Experimental neuroanatomical, physiological and behavioral methods applied to adult avian and reptilian brains have finally clarified several fundamental questions regarding the origins of 'neocortex' and have indicated that these origins can be viewed as consequent to two separate events: (1) The elaboration of constituent neuronal populations and their associated connections that are common to the telencephalae of both nonmammalian and mammalian amniotes. In mammals these populations are found within the so-called neocortex. In birds and reptiles, most of these neurons are found within the dorsal and dorsolateral ventricular ridges (DVR and DLVR). (2) In mammals, the components of the DVR and DLVR are incorporated into the thin overlying pallium to form a laminated 'neocortex'. Analysis of development in domestic chicks suggests that the DVR is one of several prosencephalic neuromeres ('Prosomeres') that contribute to the ontogeny of comparable structures in birds. Perhaps in mammals, as well, cortical development is consequent to incorporation of these several prosomeres into proliferative ependyma of the pallial mantle.