Expression of jak2a is perturbed in embryos with mutation in the spadetail gene. Clutches of embryos derived from heterozygous spt parents were raised for 14 and 24 hours and assayed for the expression of stem cell and erythroid marker genesscl (b, d, f, and h) and jak2a (a, c, e, and g), respectively. The spt mutant can be unambiguously scored at the developmental stages presented here by the loss of trunk somites; embryos shown in (c), (d), (g), and (h) are spt homozygotes. Note the loss of jak2a staining at all stages in thespt mutant embryos. Embryos in (a) through (d) are dissected and flat mounted, with the anterior to the left; all other embryos (e) through (h) are displayed in a lateral position with anterior to the left and dorsal to the top of each panel.