Asymmetric cortical areas differ in volume and in the number of neurons. There are also differences between asymmetric and symmetric areas. As asymmetry increases, the total area of the region decreases, suggesting that when a brain is symmetric, it is the result of two large sides rather than two small sides. Also, these volume differences are caused by changes in the number of cells, not changes in cell-packing density. The ontogenetic basis for this difference in cell numbers likely relates to events that occur quite early in corticogenesis before final mitosis of proliferative units, but definitive proof is lacking. Finally, the pattern and degree of callosal connections differ between symmetric and asymmetric brains, with differential axonal pruning being implicated as the likely mechanism.