The number of neurons in the neocortex is the product of the size of the preneurogenetic founder population, that is, the number of proliferative cells that are present at the onset of neurogenesis, and neurogenetic amplification occurring as neurons are being produced. The amount of neuronogenetic amplification is determined by changes in the output fraction, Q, from 0 to 1, over a fixed number of cell cycles. Greater neurogenetic amplification would occur across species if the number of cell cycles during which Q < 0.5 increased. Since neither the length of the cell cycle nor the length of the neurogenetic interval, that is, time per se, influence neuron number directly, it is speculated that changes in these parameters are essential to neuronal diversity.
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