Fig. 1. Development of the carapacial ridge in turtle embryos. (A,B) Comparison of the external morphology of TK stage 14 P. sinensis (A) and HH stage 26 chicken (B) embryos corresponding to the late pharyngula stage. Enlargement of the flank is shown on the right. In both of the embryos, longitudinal ridges appear on the lateral aspect of the flank (arrowheads). The ridge in P. sinensis represents the CR. (C) Comparison of transverse sections of a chicken-quail chimera (left), in which the somite has been replaced with that of a donor quail, and the HE-stained P. sinensis embryo (right), at comparable stages [for comparison of the developmental stages, see Nagashima (Nagashima et al., 2005)]. The longitudinal ridge in the chicken represents the Wolffian ridge caused by the folding of the proximal portion of the lateral body wall, or the somatopleure, which is also seen in the turtle. Arrows indicate the junction of the lateral body wall and the axial part of the embryonic body. Note that this junction corresponds to the boundary between the somite-derived and lateral-plate-derived dermis in the chimera. (D) Comparison of the developmental sequences of chicken (left column) and P. sinensis (right column) embryos. Transverse sections of the flank are shown for both animals. In the chicken, the indentation at the axial-lateral body wall junction (arrows) flattens out as development proceeds, whereas in P. sinensis, the junction remains visible because of the development of the CR, which arises just dorsal to the junction. Thus, the CR represents a ventrolateral limit of axial structure probably containing somite-derived mesenchyme. Scale bars: 1 mm for A,B; 100 µm for C,D. cr, carapacial ridge; hm, hypaxial muscles; r, rib; WR, Wolffian ridge.