Early neurogenesis.
After the symmetry of the early embryo is disrupted by morphogenetic movements, one of the first differentiation events is the appearance of discrete neural precursors, the neuroblasts, which segregate from a defined region of ectoderm. Within this region, presumptive epidermal precursors are intermingled with the neuroblasts. Sorting between these fates appears to depend upon cell-cell interactions and requires the function of two sets of genes; one whose mutant phenotype results in an excessive central nervous system (CNS) and the other a subnormal CNS. Molecular cloning of these genes reveals striking homologies which can place the products into two camps. The first are recognizable cell surface components (and potential signal transducers), and the second DNA binding proteins, plausibly transcription factors.