Fig. 1. Definition of the levels of a forming boundary in the anterior PSM as seen in sagittal section of an E2 embryo under Nomarski optics. Anterior is at left. (A) The first feature of morphological segmentation is a gap within mesenchymal PSM, and this position is designated as the line 0. The site located one-somite length posterior to the line 0 is defined as the level –1. The border cells immediately anterior to the gap undergo epithelialization resulting in a solid boundary at the line +1. (B) Identification of the line 0 in living embryos was confirmed by the location of the condensed cell mass posterior to +1. The position of level +1 was judged by combining a landmark made by a GFP/COS implantation and histological criteria. GFP in a paraffin section was visualized by anti-GFP antibody (brown).

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