Fig. 2. spt and ntl are required together for trunk and tail mesoderm development. (A-D) Live wild-type and mutant embryos at 24 hpf. Embryos doubly homozygous for spt and ntl mutations (D) have more severe posterior mesodermal deficiencies than ntl^– (B) and spt^– (C) embryos. Anterior development appears unperturbed. (E-H) Tranverse sections through trunk regions of wild-type (E) and mutant embryos (F-H) at 30 hpf show that (F) ntl^– embryos lack a differentiated notochord, (G) spt^– embryos have reduced muscle tissue, and (H) spt^–;ntl^– embryos lack all recognizable mesoderm, having only a spinal cord covered by skin. sc, spinal cord; no, notochord; sm, somitic muscle; *, pronephric tubule. Scale bars, 250 µm (A), 25 µm (E).