Molecular Genetics of Drosophila Neurogenesis B. YEDVOBNICK,*: ~ M.A.T. MUSKAVITCH,t K.A. WHARTON, * M.E. HALPERN, * E. PAUL,*

The central nervous ystem in Drosophila is derived from a set of embryonic precursor cells called neuro-blasts. Morphogenetic movements are initiated imme-diately after blastoderm formation, resulting in the es-tablishment of the ectodermal nd endodermal germ layers. Subsequently, the morphologically homoge-neous ectodermal layer exhibits he first visible signs of differentiation (Poulson 1950; Hartenstein and Campos-Ortega 1984). The region of the ectoderm be-low the mesoderm and lying symmetrically on both sides of the ventral midline consists of approximately 1600 cells and is designated the neurogenic region. As development proceeds, cells within this medial ectoder-mal region uniformly increase in size, rendering them morphologically distinct from the remainder of the ec