NB 3-1 T

Segregation wave: S3

The NB 3-1 lineage consists exclusively of neuronal cells in both thoracic and abdominal neuromeres. It comprises the four contralaterally projecting motoneurons RP1,3,4,5 (Goodman et al., 1984; Sink and Whitington, 1991) . In addition to these motoneurons the clone includes a cluster of interneurons which are positioned in the ventral cortex region (3-1I). The lineage differs in the thoracic and abdominal neuromeres by the number of the interneurons and the arrangement of their cell bodies in the cortex. Since at stage 17 we generally find labelled cell fragments associated with the abdominal but not the thoracic NB 3-1 clones the lower average number of cells in the abdominal clones may be due to cell death. We cannot exclude that cell death occurs at later stages in the thoracic lineage. Furthermore, the contralateral fascicle of the abdominal 3-1I bifurcates in an anterior-posterior direction, after traversing through the anterior commissure, whereas at the same stage this bifurcation is not obvious in the thorax. This difference might also be due to a temporal delay in fibre growth in the thorax. The dorsal group of RP-motoneurons (RP1,3,4) forms a distinct fascicle along the posterior border of the anterior commissure. The axon of the RP5 which is positioned ventrolaterally to the dorsal group (Sink and Whitington, 1991) extends first along the interneuronal fascicle and joins the RP-fascicle inside the anterior commissure. The common RP fascicle leaves the nerve cord through the anterior root of the intersegmental nerve. The observed innervation pattern of ventral and ventrolateral muscles is consistent with previous descriptions (Sink and Whitington, 1991) .

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