Embryogenesis of an insect nervous system II: A second class of neuron precursor cells and the origin of the intersegmental connectives

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Abstract
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 segmentaliy repeated pattern. This and the standard pattern of axon outgrowth in different segments suggest that the nervous system develops to a common, segmentaliy repeated programme.