Fig. 4

Development of secondary loops in the small intestine during the 7^th and 8^th week is invariant. Right-lateral (panels a1-c1) and caudal (panels a2-c2) views of the reconstructed intestines of CS18 (s97; panel a), CS20 (462; panel b), and CS23 (s4141; panel c) embryos. The beige strand (8) represents the periductal mesenchyme. Ovals: umbilical orifice. Note the dorsal (a) and then leftward growth (b, c) of the apex of the 1^st secondary loop (coded red and orange), followed by the formation of tertiary loops in its distal limb (c; coded orange). Further note the caudal (a) and then leftward growth (b, c) of the apex of the 2^nd and 3^rd secondary loops (coded yellow and green, respectively), followed by the formation of tertiary loops (c). Also note that the apex of the 4^th secondary loop (coded blue) grows cranially (a) before forming tertiary loops (c). Scale bar units: μm. An interactive 3D-PDF is available online (3D-PDF CS20)