Embryonic neurogenesis in the ventral nerve cord of the locust Schistocerca gregaria was studied using toluidine blue (TB) staining and birthdating of cells by incorporation of bromodeoxyuridine (BUdR). In the thorax, the neuroblasts (NBs) start dividing at the 28% stage and neurogenesis continues until 90%. In the abdomen, neurogenesis starts at about 30% and continues until 70%. Every NB appears to have its own fixed period of division before disappearing. Thus a specific spatial and temporal pattern of NB degeneration can be seen in every segment. This pattern is identical in each of the three thoracic ganglia. We have traced the fate of each of the original complement of NBs in the mesothoracic neuromere and specified the stage of development at which each NB ends its lineage and disappears. The abdominal segments A2 to A7 share an identical pattern of NB death, which is not comparable to the thoracic pattern. The progress of neurogenesis is marked by a gradual decrease in the number of NBs, with NBs in the thoracic ganglia persisting longer than their abdominal homologues. The differences between the thoracic and abdominal NBs are also reflected in the rates at which they divide, thoracic NBs dividing at almost twice the rate of the abdominal NBs.

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