Fig. 2.

Abdominal wall muscles appear in horizontal sections of a 6-week embryo (CRL 16 mm). HE staining (panels A-C) and silver impregnation (panels D-F). Panel A (or Panel F) is the most superior (or inferior) level of the figure. The rectus abdominis (R) appears to be differentiated at levels above the umbilicus (panels A and B), but it is identified as a mesenchymal cell cluster alongside the umbilicus (panels C and D) and is difficult to identify below the umbilicus (star in panels E and F). Likewise, the oblique muscles (EO, IO) are not identified below the umbilicus (panels E and F). The rectus abdominis is located in the lateral part of the body wall rather than the anterior site. The rectus sheath (arrowheads) was not seen at any level. The diaphragm has not yet developed. The future inferior part of the pleural cavity is filled with loose tissue (PC in panels A-C). All panels were prepared at the same magnification (scale bar in panel B). In all figures including Fig. 1, the upper side of each panel corresponds to the anterior or ventral side of the body. AO, aorta; SA, serratus anterior; SMA, superior mesenteric artery; UA, umbilical artery; UR, urachus; EO, external oblique; IO, internal oblique; PC, primitive pleural cavity.