Fig. 1. (A) Surround repulsion of peripheral DRG axons in the chick embryo trunk. Schematic diagram through the trunk in transverse section showing tissues that are sources of secreted repulsive signals (red arrows). These guidance cues operate in concert to constrain DRG axon paths in a typical bipolar trajectory (black arrows). (B) Expression of SEMA3A in the chick embryo notochord. Following whole-mount in situ hybridisation on stage 19 embryos, transverse sections (30 µm) were cut and mounted. The section shown is through the brachial region and SEMA3A expression is seen in the notochord and dermamyotome. Note that the neural tube split open during the hybridisation procedure. dm, dermamyotome; drg, dorsal root ganglion; fp, floor plate of neural tube; nt, neural tube; n, notochord; sc, sclerotome. Scale bar: 100 µm.