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Fig. 4 Distribution of Zf-T mRNA in early embryos. (A-D) Schematic drawings of embryos, indicating (shaded) the distribution of Zf-T mRNA as revealed by whole-mount in situ hybridizations. Embryos are oriented with their animal pole up, and with their dorsal side to the right (in A, the dorsal-ventral polarity is not discernable yet). (A) Sphere stage, 4.0 hours. The blastomeres sit on top of the yolk (Y), epiboly is just about to start. (B) Embryonic shield stage, 6.0 hours. Epiboly has led to a spreading of cells over the yolk mass, which has undergone a change in shape. Involution causes a thickening of the embryo around its equator. Dorsal convergence leads to a thickening of the germ ring (GR) at the dorsal side, the embryonic shield (ES). (C) 80% epiboly, 8.0 hours. The embryo is two-layered: cells that have involuted constitute the hypoblast (HY; future endo- and mesoderm), cells that have not involuted constitute the epiblast (EP). (D) 2 somite-stage, 11.0 hours. Epiboly has finished, but gastrulation continues in the tailbud (TB) which is going to extend over the next couple of hours to make up the posterior half of the body. (E-G) Whole-mount in situ hybridizations using antisense RNA probes to reveal spatial expression of Zf-T. (E) Embryonic shield stage. Cells in the region of the germ ring express high levels of Zf-T RNA. Note the thickening at the dorsal side, the embryonic shield (ES). (F) Tailbud stage. Zf-T RNA is expressed in the presumptive notochord and in the tailbud (TB), but not in cells of the paraxial mesoderm. The staining is confined to the hypoblast and cannot be detected in any cells of the epiblast (EP). (G) 5-somite stage. Expression is not uniform along the anteriorposterior axis, but highest in cells of the posterior presumptive notochord. Anterior at the top.