Fig. 2. The OP lineage of rostral segments is serially homologous to the sum of the O and P lineages in more posterior segments. This embryo was fixed and dissected at stage 9, by which time the primary blast cells have given rise to their segmental clones of differentiated descendants. The op₄ clone is labeled with rhodamine, and the more posterior O and P lineages are labeled with rhodamine and fluorescein (see Materials and methods). The preparation shown here is oriented with anterior to the top, ventral midline to the left and dorsal to the right. The segmental boundaries are marked on the left side of each panel. (A) The morphological pattern of differentiated descendants in the op₄ blast cell clone (red) is overtly similar to one segmental repeat of the O and P lineages combined (yellow). (B) The op₄-derived pattern elements in the ventral nerve cord are homologous to the segmental pattern elements derived from the O and P lineages. The ventral nerve cord is visualized with Hoechst 33258 counterstaining (blue). Scale bar: 20 µm.