Studies on the mechanics of development of the visual pathways in the chick embryo

Silver-stained whole mounts of the tectal surface were used to study the developing retinotectal fiber pathway in the chick embryo. The growing front of optic fibers appeared more disordered than fiber bundles to the rear. The fiber pattern as a whole appeared more orderly with age. Some fibers projected sparsely ahead of the growing front in a pattern suggesting the existence of preneural guidance channels. Fiber branching was rarely seen. An orthogonal gridwork of two layers of fibers, running roughly anteroposteriorly and dorsoventrally, was found on the developing tectal surface after removal of both optic vesicles. In unoperated specimens, fibers in the optic fiber layer followed these grid lines.Retinal quadrants from embryos 4–5 days old were transplanted to the optic tecta of embryos 6–7 days old. The graft fibers subsequently did not show specific orientation toward their appropriate tectal quadrant. Rather, the fibers followed the same straight courses taken by carbon particles implanted in comparable positions in controls.After the production of quadrantal retinal lesions in 4–5 day-old embryos, no evidence was found for specific tectal innervation defects at 12–13 days. Lesions, irrespective of retinal quadrant, resulted in a relative lack of innervation in the posterodorsal aspect of the 12–13 day-old tectum. This was probably due to a delay in the advancement of the growing front of fibers across the tectal surface. The results weaken previous support for specificity in the guidance of developing optic axons.