Fig. 1. (A) Schematic presentation of the X_zY_zZ_z and X_fY_fZ_f coordinate systems. The X_zY_zZ_z coordinate system in blue is associated with the optical sections taken by the CLSM. The X_fY_fZ_f coordinate system in red is defined relative to the fish body. (B) Dorsal view of the postanal part of a fish embryo, anterior to the top. The body axis is indicated by a black line. Muscle fibres are indicated by black arrows and run in a particular direction in the left half of the body. The direction of muscle fibres in the right half of the body is the mirror image of that in the left half. When the angle of sectioning is perpendicular to the body axis, the observer (red eye) looks at right angles to the sections that are perpendicular to the body axis (red lines). When the sections are not made at an angle of 90° to the body axis, the observer (blue eye) still looks at right angles to the sections (blue lines), but these present the fibres as if they run asymmetrically (C). (D) Rotating the section mathematically in such a way that the view of the red observer is obtained will yield the correct symmetrical muscle fibre field shown in E. Dotted lines indicate parts of muscle fibres and body axis that are not covered by the sections.