Labeling of the retino-fugal axons shows the major components of the chick visual system projections, as indicated in the schematic (D). Injection of 4Di-10ASP into the right eye at E4 resulted in anterograde labeling of axons in the anterior pole of the left tectum (A corresponds to intersection A of the diagram in D). As expected from former studies (O’Leary et al., 1983, Thanos and Bonhoeffer, 1984), axons were also labeled in the anterior pole of the ipsilateral tectum (B). C, Anterograde staining of axons (arrows) that terminate in the noninjected contralateral retina and of cell bodies (arrowheads) whose axons have reciprocally projected into the injected right retina. The optic fissure (OF) at the top is brightly fluorescent because of a background autofluorescence, which is independent of injection. Note that the anterogradely labeled axonal tips occupy the more temporal (te) half of the retina, whereas retrogradely filled ganglion cell bodies are predominantly in the nasal (na) part. D, Summarizing schematic showing the experimental setup and indicating the positions photographed in A–C. Scale bar, 100 μm.