Fig. 5

Fig. 5 Serial sections of whole-mount embryos depicted in previous figure. The embryos shown in Fig. 4 were embedded in paraffin and cross-sectioned. Neighbouring sections were either observed with Nomarski optics directly (A, C, E, G, I, K. M) or after staining with haematoxylin and eosin (B, D, F, H, J, K, L, N). Embryos dorsal side oriented up. (A and B) Sections through the head of an embryo at the 90%, epiboly stage. Expression is confined to three to five cells located superficially at the edge of the neural plate thickening. (C and D) Sections through the head of a Ibsomite embryo. Expression is observed as a scattered staining within the epidermis covering the brain and the eyes. Weak expression is seen in the ventral diencephalon. (E and F) Sections through the tailbud region of the same embryo. Expression is detected in the dorsal part of the neural tube which will generate the roof plate and the dorsal fin. (G and H) Sections through the head of a 2bsomite embryo. at the level of the eyes. Radar expression is confined to the dorsal part of the retina. Only the upper half of the sections are shown. (I and J) In the posterior trunk of the same embryo, at the level of the yolk tube, staining is detected in the hypochord (arrowhead), the neural tube and loosely connected cells located between the somites, the hypochord and the gut. These cells may undergo migratory movements. Expression is also detected at three positions within the epidermis surrounding the yolk tube. (K and L) In sections of the anterior trunk from the same embryo, expression is detected only in the roof plate. Only the upper half of the sections is shown. (M and N) Sections at the tail level from the same embryo reveal low levels of expression in the dorsal and ventral fins. Stronger expression is detected in the hypochord and in the median mesenchymal region immediately ventral to it.