A single cycle of eve1 expression. A-D: Embryos very close in developmental age have been ordered from top to bottom according to the number of visible LBS (as indicated in black on the left) and the segmental position of the most recently intercalated eve1 stripe (as indicated in blue on the left, and marked on the embryos with blue arrows). Together these markers provide an independent estimate of the developmental sequence for these four embryos. The series shows the oscillation of eve1 expression in the periproctodeal region that generates the initial double periodicity stripe of eve1 corresponding to the 28th LBS, a period that corresponds to the formation of two segments. Each row presents three views of the same embryo as in Figure 2. F: Montages showing halves of embryos B3, C3 and D3 abutted with embryo A3 to show how expression across the peri-proctodeal area expands while the external ring corresponding to the 24th LBS (marked by red arrows) shows no detectable movement over this time window. E: Posterior view of an embryo showing bilateral asymmetry in the stage of the oscillation for eve2, resembling the juxtaposition of A3 and B3. Scale bar: 400 μm. All embryos with ventral sides at the bottom.