jak2b expression in the developing zebrafish. The expression of jak2b in embryos at various developmental stages was examined by whole mount in situ hybridization. All embryos and larvae are shown with anterior to the left and dorsal to the top of the frame. (a) 24 hpf embryo, lateral view showing detection of jak2b message restricted to the lens of the eye (arrow) and in the nephritic ducts (arrowhead). (b) 24 hpf embryo, ventro-lateral view at higher magnification, showing the region of the nephritic duct stained by the jak2b riboprobe. The bilaterally symmetric nephritic ducts are visible (arrows) as is the distal tip of the ducts at the proctodeum (arrowhead). (c) 48 hpf embryo, lateral view showing expression of jak2b persisting in the lens of the eye (arrow) and decreasing from earlier levels in the nephritic ducts (arrowhead). (d) 8 dpf larva, lateral view at higher magnification, indicating detection of a low level ofjak2b message in elements of the jaw (arrow), in the developing gills (bracket), and in the anterior lateral line (arrowheads). Expression of jak2b in the posterior lateral line is evident, but is not shown in this figure.