Figure 1. Spiral cleavage of a leech embryo as viewed from the animal pole. The shaded area is a pool of yolk-free cytoplasm called teloplasm. The first and second cleavage planes fall parallel to the animal–vegetal axis (i.e. line of sight), and divide the embryo asymmetrically such that the D macromere inherits the teloplasm. Spiral cleavages begin at the third round of cell division, and involve highly asymmetric cleavages that produce a much smaller daughter cell or micromere at the animal pole. The orientation of these spiral cleavages alternates from clockwise to counterclockwise (arrows) at successive divisions. Note that during the fourth cleavage, the D¢ macromere differs from the other macromeres in that it cleaves symmetrically to produce two large cells. The DNOPQ cell inherits the pool of animal pole cytoplasm and serves as the precursor of the definitive segmented ectoderm, whereas the sibling DM cell inherits the pool of vegetal pole cytoplasm (not shown) and serves as precursor of the definitive segmented mesoderm.

Figure 2. The leech body plan is subdivided into an unsegmented head region called the prostomium – which is of micromere origin – and segmented tissues arising from the teloblasts. There are five teloblasts on either side of the embryo (only shown here for the left). Each teloblast undergoes repetitive stem cell divisions to generate a long column of primary blast cells, and the five columns of blast cells merge together in parallel to form a germinal band. The N, O, P and Q teloblasts generate ectodermal blast cells that lie on the outer surface of the germinal band, and the singular M teloblast generates mesodermal blast cells that lie underneath. The production of blast cells by the teloblast is protracted, and this illustration shows a developmental gradient in which only the first-born blast cells – which lie furthest from the teloblasts – have differentiated to form the most anterior segments. Blast cells that were born at progressively later times are shown at earlier stages in their differentiation, and will eventually contribute to successively more posterior segments.

Figure 3. Leech embryos undergo gastrulation by epiboly, shown here for the genus Helobdella. The micromere cap (grey) originates on one side of the animal pole (An), and then spreads out over the surface to engulf the remainder of the embryo. Images in the upper row are left side views of the embryo, with the B quadrant to the left and the D quadrant to the right. The images in the bottom row are rotated 90° about the animal–vegetal axis, and hence show views from the B quadrant side of the embryo. Throughout epiboly, the germinal bands (blue) lie along the edge of the expanding micromere sheet. The anterior end of the germinal band remains associated with the animal pole and the posterior end remains close to the teloblasts (TBs). Thus, as the germinal band elongates (by addition of new blast cells at its posterior end) it sweeps over the surface of the embryo until meeting and fusing with the contralateral germinal band on the far side of the embryo. Germinal band fusion occurs around roughly 270° of the embryo's circumference, and line of fusion marks the future ventral midline.