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Fig. 3. Developmental patterning of the CR. (A) Schematic
representation of the development at the trunk level in both species. Chicken
and P. sinensis embryos show similar developmental patterns up to
common stage 6 (Nagashima et al.,
2005), when the proximal part of the lateral body wall swells
laterally over the surface of the embryo at its junction with the axial part
of the body (arrows), forming the Wolffian ridge. This junction is seen as an
indentation on the lateral surface of the embryo, as is seen commonly in the
amniote pharyngula. In the chicken embryo from HH stages 22 to 25,
sclerotome-derived rib primordia and muscle plate invade the body wall, first
as abaxial elements, and from stage 26 onwards, dorsal ribs and related
muscles invade the body wall as primaxial elements. However, in P.
sinensis, only the poorly developed hypaxial muscle is recognized as an
abaxial element in the body wall, and axially developed ribs and muscles,
although primaxial elements, never invade the body wall. The ventrolateral
part of the axial domain also swells to form the CR adjacent to the Wolffian
ridge dorsally. In the chicken, the lateral surface of the embryo flattens in
later development. (B) Schematic representation of the axial domain and
the lateral body wall in amniote embryos. (C). Differences in the
expansion of the primaxial elements between turtles and other amniotes. Note
that the muscle tissues are omitted from this scheme, and the lateral somitic
frontier is shown by dotted lines. The primaxial ribs in amniotes correspond
to the dorsal ribs of vertebrates; these arise initially in the axial domain
of the embryonic body and grow secondarily ventrally into the body wall. In
the turtle, the carapace is made of ribs that fail to invade the body wall.
Thus, the growth pattern of the ribs and the primaxial dermis are co-extensive
in the turtle. c=dr, carapace=dorsal ribs; da, dorsal aorta; dr, dorsal rib;
hm, hypaxial muscle; lsf, lateral somitic frontier; mp, muscle plate; n,
notochord; na, neural arch; nt, neural tube; sc, sclerotome; str, sternal rib;
WR, Wolffian ridge.
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