The origin of the central nervous system during embryogenesis in Drosophila melanogaster depends upon the proper segregation of neuroblasts from the primative ectoderm and the subsequent differentiation of neuroblast-derived neurons into the segmentally organized nervous system. We have recently been able to isolate embryonic neuroblasts from gastrula-stage Drosophila embryos insufficient quantities to use as an antigen for preparing monoclonal antibodies and to examine neural specific genes and gene products. In addition, we have identified two female sterility mutations which affect the early embryonic determination step at which ectoderm cells become either neuroblasts or epithelium. We will prepare monoclonal antibodies against freshly isolated neuroblasts and from neuroblasts as they differentiate In Vitro into recognizable neural nets. We will also prepare cDNA probes to poly (A) containing RNA of differentiating neurons In Vitro to screen genome libraries of neural specific genes. Finally, we will clone the genes for the two female sterility mutations affecting neurogenesis. With these probes we will analyze the genetic organization of the major genes involved in neurogenesis and we will use the monoclonal antibodies as tools to investigate the development and organization of the embryonic nervous system. These investigations will provide a model for the understanding of early developmental switches and for the early differentiation of complex nervous systems.