The embryonic development of the central nervous system (CNS) involves the generation of an enormous diversity of cellular types arranged and interconnected in a remarkably precise pattern. In each hemisegment of the grasshopper embryo, the ectoderm generates a stereotyped pattern of 30 neuronal precursor cells, called neuroblasts (Fig. 1)1. Each of these stem cells makes a stereotyped contribution of 6-100 progeny to the ∼1,000 different neurones, each cell identifiable according to its unique morphology, physiology and biochemistry. What are the contributions of cell interactions and cell lineage to the generation of this diversity and specificity of identified neurones in the grasshopper CNS? Here we report on cell ablations with a laser microbeam at different stages of development. Our results suggest the importance of cell-cell interactions in the determination of ectodermal cells to become identified neuroblasts2. However, once a neuroblast begins to divide, then cell lineage appears to play an important role in the determination of its stereotyped family of neuronal progeny3. Furthermore, cell-specific interactions continue to play an important role as neurones, according to their mitotic ancestry, recognize and interact with other differentiating neurones in their environment4-6.