Figure 6

Relative positions of axillary sites in the pilidium. Left: diagrammatic pilidium in side view; apical organ (ao) and episphere (ep) up, lappets (la) down, primary ciliated band (pcb) defining larval margin. Inside are stomach (st) and buccal funnel (bf), divided from exhalant gutter (ex) by buccal ciliary ridges (br). Colored patches indicate axils, both outer (bold) and inner (faded), in the anterior (green), posterior (blue), and, hypothetically, at the end of the buccal ciliary ridges (purple). Darker circles within axils indicate those which give rise to imaginal discs. The fact that the hyposphere of the pilidium is almost entirely folded into the episphere at gastrulation obscures the relationship between axils; therefore, to the right we invite readers to imagine a spherical pilidium, that is, as if the blastocoel were inflated. Once the hyposphere is thus everted, such that the entire contiguous ectoderm and endoderm are laid out upon the surface of a sphere, the symmetry of the relation between anterior and posterior axils with respect to ciliated band and imaginal rudiments becomes plain. Again, the question mark indicates a hypothetical axil at the end of the buccal ridges; the cerebral organ discs originate at the end of the buccal ridges, and we detect both label dilution (Figure 1B, C) and BrdU incorporation (Figure 3A, B arrowhead 2, also Figure 3C) in these areas, but we have not spotted the expected population of uniciliated cells beneath the dense ciliation where the inner band of the lappets meets the end of the buccal ridges.