Evolution, development, and initial function of the mammalian neocortex: response of the germinal zones to endothermy

In the mouse the release of neocortical neurons from the periventricular germinal layers of the forebrain commences towards the ventral margin of the lateral pallium at the level of the interventricular foramen and is propagated from there across the lateral wall of the hemisphere. In the adult cortex the origin of the gradient corresponded to the ventral portion of the somatotopic map of the body, that is, to the area representating structures derived from the embryonic branchial arches, namely, the peri-oral region and laryngo-pharyngeal masticatory apparatus. Branchial arch nerves also innervate the fore- and mid-gut and all the related exocrine and endocrine glands. This suggests that the mammalian neocortex evolved from a visceral integration area in a positionally equivalent area in the pallium of a reptilian ancestor which expanded in relation to extensive changes taking place in the visceral and branchial systems of the body during the transition from reptilian ectothermy to mammalian endothermy. The practical problem facing early mammals was to acquire and process the extra energy required to sustain a continuously high metabolic rate. Improvements to the food processing capabilities of the visceral and branchial systems and the expansion of their neural control were important components in the conglomerate of changes required to sustain the increased energy demands of endothermic tissues. Endothermy also bestowed the ability to sustain greater numbers of metabolically expensive neurons and this, in turn, required an appropriate response from the cell production mechanisms in the periventricular germinal layers.