Fig. 1. Early development of the OP lineage. (A) The OP proteloblast undergoes four asymmetric cell divisions to form the four primary op blast cells. The op₄ blast cell is labeled red. The OP proteloblast undergoes a symmetric cell division to form the two O/P teloblasts. Each O/P teloblast then resumes asymmetric cell divisions to form the o and p bandlets. Blue arrows indicate the axis of cell division. (B) Divisions of the op₄ blast cell. (C) Fluoromicrograph showing the four tertiary blast cells of the op₄ clone (clonal age is 44 hours). The OP proteloblast was injected with rhodamine dextran immediately before the birth of the op₄ blast cell. The specimen was counterstained with Hoechst 33258, and is shown with anterior toward the top. The boundary between the op₄ clone and the o and p bandlets is marked by a yellow line. Note that the posterior edge of the op₄.pp cell retains a unique chevron shape which borders the o bandlet to the future ventral side (left) and the p bandlet to the future dorsal side (right). Scale bar: 10 µm.