Fig. 5. The neural tube specifies the lateral rectus eye muscle. (A) Schematic dorsal view and (Ai) cross section (dorsal to the top) illustrating the surgical procedures at HH10; head mesoderm in pink, the position of the impermeable barrier (tantalum foil) is indicated. (B,E,H,K,N) Lateral views of unoperated sides; anterior to the top, dorsal to the right. (C,F,I,L,O) Lateral views of operated sides; anterior to the top, dorsal to the left. (D,G,J,M,P) Cross sections at anterior hindbrain/mandibular arch levels; dorsal to the top, operated sides to the right, markers and the position of the barriers are indicated. Note that upon separation of the neural tube from the head mesoderm, the lateral rectus markers Lbx1 (B-D) and Paraxis (E-G) failed to be expressed. Myf5 (H-J) and MyoR (K-M) expression continued. En2 expression (N-P) spread form the mandibular arch muscle anlage medially, encompassing the muscle normally expressing the lateral rectus markers. Thus, muscle specification, but not differentiation, was perturbed. Scale bar: 200 µm. d, diencephalon; da, dorsal aorta; ect, surface ectoderm; fge, foregut endoderm; fp, floor plate of the neural tube; ht, heart; hy, hyoid arch; m, midbrain; ma, mandibular arch; ncc, neural crest cells; not, notochord; nt, neural tube; ov, otic vesicle; r, rhombomere; V, trigeminal ganglion; VII, facial nerve.