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The intersegmental connectives in the locust central nervous system are initiated by the axons of early differentiating neuron trios. Using a combination of electron microscopy and fluorescent dye injection we have shown that the axons of these cells grow out anteriorly and posteriorly in each segment along a basement membrane, and link together at the segment borders to form continuous longitudinal pathways on each side of the developing nervous system. These early neurons are the progeny of a second class of precursor cell, the midline precursors, which are distinct from the segmental neuroblasts. Like the neuroblasts, the midline precursors are arranged in a standard segmentally repeated pattern. This and the standard pattern of axon outgrowth in different segments suggest that the nervous system develops to a common, segmentally repeated programme.

Maps of the thoracic and abdominal neuroblasts have been prepared by reconstruction from serial sections of timed Locusta migratoria embryos. The maps are bilaterally symmetrical, periodic and consistent for embryos of the same age, with a fundamental arrangement of 7 rows of 4-10 cells plus 1 median neuroblast per segment. A map of 60 + 1 cells is repeated in each of the three thoracic segments, with an additional median cell developing late at the anterior end of the prothorax. The arrangement in the abdomen is similar, with 56 + 1 cells per segment. Neuroblasts differentiate and subsequently degenerate in an antero-posterior sequence, but construction of the thoracic ganglia involves a delayed degeneration of part of the original set in these segments. The maps show that the neuroblasts are as reliable in their number and arrangement as the adult cells they produce. The number of neuroblasts used in constructing the relatively complex thoracic ganglia is similar to that which produces the simpler abdominal ganglia. Some motorneurons have the same parent neuroblast. The maps are intended as a first step in an analysis of the relation between the progeny of different neuroblasts and the family of neurons which each neuroblast produces.