Fig. 1. Variability of hairy1, hairy2 and lunatic fringe expression patterns is evident at the level of sinus rhomboidalis. In situ hybridisation with the hairy1 (A-C), hairy2 (D-F) and lunatic fringe (G-I) antisense mRNA probes, showing the variety of expression patterns observed at the level of PSM and sinus rhomboidalis, in six-somite stage embryos. (Left) Electron scanning micrograph of the sinus rhomboidalis of a six-somite stage embryo (dorsal view). The lines define the AP levels of the transverse sections. (A1-I1) Cross-sections at the level of the posterior limit of the median pit. (A2-I2) Cross sections 80-100 µm caudal to the posterior limit of the median pit. (A3-I3) Cross-sections 160-180 µm caudal to the posterior limit of the median pit. (A4-I4) Cross-sections 240-260 µm caudal to the posterior limit of the median pit. We consider the median pit to be the region that comprises the pit itself and the slope surrounding it. A transverse section at the same level, in two different embryos hybridised with the same probe, evidences distinct patterns of expression (e.g. A2-C2). Transverse sections at adjacent levels also evidence distinct patterns of expression, within the same embryo (e.g. G2-G3). PM-PSM, prospective medial–presomitic mesoderm. Rostral is towards the top. Scale bar: 250 µm in whole-mount embryos; 110 µm in cross sections.