Fig. 1. Whole-mount double in situ hybridisation reveals two blood and endothelial progenitor populations during early Xenopus development. Overlapping expression patterns of blood (SCL) and endothelial (Xfli1) genes suggest the existence of two blood and endothelial progenitor populations in the early Xenopus embryo: one in the anterior ventral mesoderm of the mid neurula embryo and another in the DLP of the early tail bud stage. Black arrows indicate DLP; red arrows indicate VBI; green arrows indicate vitelline veins (Vit). In all panels, numbers in the top right-hand corner indicate stage of development. 17 and 20a, anterior views; 20, 22b ventral veiws; 20b, dorsal view; 22a, 23, 24, 26,28 and 31, lateral views. Anterior is towards the left with exception of 17 and 20a. (A) Time course from stage 17 to 28 showing SCL expression. Notice that SCL expression in the DLP is first detected at stage 24 (black arrow). (B) Time course from stage 17 to 28 showing Xfli1 expression. Notice that Xfli1 expression in the DLP is first detected at stage 20 (black arrows). (C) SCL (turquoise) + Xfli1 (purple) whole-mount double in situ hybridisation in early Xenopus embryos. Black arrowheads indicate SCL⁺Xfli1^- cells. Green arrows indicate the developing vitelline veins (Vit) surrounding the VBI (red arrows). NF, neural fold; CG, cement gland.