The precursors of the CNS derive from specialized parts of the ectoderm, the neurogenic regions, which are show here projected on a stage 5 embryo. The ventral neurogenic region gives rise to the neuroblasts of the ventral nerve cord, the part of the CNS belonging to the segmented germ band. The precursors of the CNS (neuroblasts) derive from specialized parts of the ectoderm, the neurogenic regions, which are shown here projected on a stage 5 embryo (see Campos-Ortega; Goodman and Doe; both this volume). The ventral neurogenic region (VNE) gives rise to the neuroblasts of the ventral nerve cord, the part of the CNS belonging to the segmented germ band (Hartenstein and Campos-Ortega 1984). Separating the ventral neurogenic region from the mesoderm (ms) is the mesectoderm (mec), a single row of cells on either side of the embryo that, among other cell types, gives rise to a number of neuronal precursors. The pro-cephalic neurogenic region (PNE) generates the brain. Adjacent to the procephalic neurogenic region is the anlage of the optic lobe (ol), which develops differently from the rest of the brain (see below). It should be emphasized that the neurogenic regions contain not only neuro-blasts, but also epidermal precursors that give rise to the ventral epidermis and the head epidermis.

Shortly after gastrulation [stage 8], the cells of the ventral neurogenic region swell. Whereas the primordium of the dorsal epidermis (DEA) undergoes its first postblastoderm division, mitosis is delayed in the neurogenic regions.

Starting at stage 9, neuroblasts delaminate from the ectoderm. For the ventral neurogenic region, delamination occurs in three waves (Hartenstein and Campus-Ortega 1984; see Campus-Ortega, this volume). The first wave (beginning of stage 9) yields two rows of neuro-blasts (SI) on either side. The neuroblasts delaminating during the second wave (SII) fill the gap between the two rows of SI neuroblasts. The figure of a stage 9 embryo depicts the pattern of SI and SII neuroblasts. SIII neuroblasts segregate from predominantly medial positions throughout stage 10 and the beginning of stage 11. The full complement of neuroblasts of a stage-11 embryo is illustrated to the left. The pattern of neuroblast segregation in the procephalic neurogenic region is insufficiently known; the drawings of the stage 9 embryo and stage 11 embryo (left) give a rough approximate of the population of procephalic neuroblasts (pnb) present at these stages.