The 9-week human embryo (approximately 63 days old) is 424% larger than the E18 rat embryo, but both embryos are near the same stage of development. The basal ganglia, hypothalamus, midbrain, pons, medulla, and cerebellum are similar in both embryos. However, the thalamus, and particularly the neuroepithelium of the cerebral cortex is much larger in the human than in the rat embryo. In humans, the cortical neuroepithelium greatly expands over a “ballooning” lateral ventricle and a “blooming” choroid plexus and is the source of most neurons that will populate the highly folded neocortex in the mature human brain. By contrast in rats, the much smaller cortical neuroepithelium covers a more flat lateral ventricle with no blooming choroid plexus , which leads to the production of many fewer neurons in the cerebral cortex of the mature rat brain .