NUMBERS, TIME AND NEOCORTICAL NEURONOGENESIS - A GENERAL DEVELOPMENTAL AND EVOLUTIONARY MODEL

The number of neurons in the neocortex is the product of the size of t he preneuronogenetic founder population, that is, the number of prolif erative cells that are present at the onset of neuronogenesis, and neu ronogenetic amplification occurring as neurons are being produced. The amount of neuronogenetic amplification is determined by changes in th e output fraction, Q, from 0 to 1, over a fixed number of cell cycles, Greater neuronogenetic amplification would occur across species if th e number of cell cycles during which Q < 0.5 increased. Since neither the length of the cell cycle nor the length of the neuronogenetic inte rval, that is, time per se, influence neuron number directly it is spe culated that changes in these parameters are essential to neuronal div ersity.