Fig. 6. Elongation of the leech body plan from teleoblasts. (A) The teleoblasts of the leech (annelid) embryo produce elongating germinal bandlets through the sequential production of blast cells by highly unequal divisions. On each side, four ectodermal bandlets (N, O/P and Q) come into lateral apposition, while the mesodermal (M) bandlet comes into apposition from beneath. The five bandlets form the germinal band, which extends across the embryo (arrow, A) as bandlet cells are added posteriorly by highly unequal cell divisions of the parent teloblasts. The germinal bands anneal with their contralateral partner in a rostrocaudal sequence at the midline to form the germinal plate. (B,C) As the germinal bandlets extend by cell division and anneal, they, and the micromere cap overlying them, expand over the surface of the embryo in epiboly (B, curved arrows, early stage 8, animal view; C, late stage 8, rostral view). (A) Reproduced, with permission, from Nelson and Weisblat (Nelson and Weisblat, 1991); (B,C) reproduced, with permission, from Smith et al. (Smith et al., 1996). Asterisks indicate apposition of germinal bands.