Expression of even-skipped and Delta genes at the onset of periodic patterning. Embryos are aligned as in Figure 1, from blastoderm stage 2.1, to the appearance of the first morphological segment, the mandible, at early stage 3.1. Panels with the same letter present different views of the same embryo (either lateral, or a view from the posterior pole orthogonal to these lateral views, with the blastopore/proctodeum at the center (black or white arrow). Ventral is to the bottom in all images. Fluorescent nuclear staining (SYBR green) for the embryos hybridized with eve1 (lateral view) and eve2 (posterior view) represents the morphology for the whole row. The temporal alignment of embryos within rows is based on double in situ for eve1 and eve2 (H, L, P, T) and for eve1 and Delta (data not shown). Note that embryo B is younger than embryos A and C. The entire pattern is dynamic at this stage, condensing anteriorly as the pattern resolves. Based on a larger series of embryos (see Additional file 7: Figure S6), we have inferred continuity of expression between stages, as for example in the comparison of K2 and O2 in this plate. Expression domains are shown in this figure with arrows corresponding to the identity of the morphological segments eventually associated with each stripe, color-coded as indicated. A lighter color has been used for stages 2.1 and 2.2, where this identification is uncertain. (ant/int - antennal/intercalary segment (apparently deriving from same initial stripe); mn, mandible; mx1, first maxilla; mx2, second maxilla; mxp, maxilliped; 4LBS, fourth leg-bearing segment). Note that in N1, R1, V1, the green arrow marks Delta expression associated with a single stripe that splits to give rise to both the maxilliped and the first LBS. Scale bar: 400 μm.