In the Drosophila embryo, the central nervous system (CNS) develops fr
om a population of neural stem cells (neuroblasts) and midline progeni
tor cells, Here, the fate and extent of determination of CNS progenito
rs along the dorsoventral axis was assayed. Dorsal neuroectodermal cel
ls transplanted into the ventral neuroectoderm or into the midline pro
duced CNS lineages consistent with their new position. However, ventra
l neuroectodermal cells and midline cells transplanted to dorsal sites
of the neuroectoderm migrated ventrally and produced CNS lineages con
sistent with their origin, Thus, inductive signals at the ventral midl
ine and adjacent neuroectoderm may confer ventral identities to CNS pr
ogenitors as well as the ability to assume and maintain characteristic
positions in the developing CNS. Furthermore, ectopic transplantation
s of wild-type midline cells into single minded (sim) mutant embryos s
uggest that the ventral midline is required for correct positioning of
the cells.