As the neural folds progressively fuse in a cranial direction, dilation of the closed neural tube occurs to form the "brain vesicles." By 3 weeks, these vesicles undergo neural segmentation and form the specific parts of the brain, that is, forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon) (see Fig. 1-7). Surface ectoderm covers the outside of the forebrain, and neural ectoderm lines the inner or facing surfaces of the paired forebrain vesicles from which the eyes develop (Figs. 1-8, 1-9). The optic sulci develop as bilateral evaginations of neural ectoderm on the facing surfaces of the
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