Fig. 1. Segmental development in Theromyzon rude illustrating the temporal progression of the main neurogenic lineage (N) and final positions of major segmental and longitudinal nerves. The relative positions of the mesodermal (M) and ectodermal (O/P, O/P and Q) teloblasts and their progeny are indicated on the right side only. Bilaterally paired N teloblasts (N_L and N_R) give rise to coherent columns of cells (n bandlets). Within the germinal plate, contralateral n blast cell clones lie in apposition across the ventral midline (broken line) and subsequently give rise to the bulk of the segmental ganglia of the ventral nerve cord, along with segmentally iterated peripheral neurons (nz1, nz2 and nz3) and a few epidermal cells (not shown). The O, P and Q teloblasts on each side give rise to distinct lineages that generate progressively more lateral and dorsal ectoderm (not shown here) (Weisblat et al., 1984). Ganglionic primordia result from the formation of transverse fissures that arise when the parent blast cell clones are 50 hours old (Shain et al., 1998). Immediately following fissure formation (50-55 hours), the first axonal processes appear between the anterior and posterior lobes of the N lineage, as evidenced by acetylated -tubulin (ACT) antibody staining (see Fig. 2). The outgrowth of two ventrolateral stripes of cells from each posterior lobe (the anterior of which expresses the leech engrailed-class gene) occurs later at 65 hours clonal age. By 100 hours, the major nerve tracts have acquired their final positions in the juvenile leech (AA, anterior-anterior; MA, medial-anterior; PP, posterior-posterior; DP, dorsal-posterior; UP, ultraposterior; C, connective; F, Faivre's nerve). Approximate clonal ages for n blast cells and their derivatives are indicated at right. Anterior is upwards. Not drawn to scale.