By means of retrograde transport of horseradish peroxidase, isthmo-optic projections were investigated in 1 to 9-day-old posthatching chickens. Some of the specimens were congenitally anophthalmic, others were monophthalmic. In contrast to a recent study, we did not detect significant differences in the one-eyed chicks. In particular, we find no indication for a retarded elimination of ectopic and aberrantly projecting isthmo-optic neurons during ontogeny. We tentatively suggest that more than one mechanism leading to monophthalmic deformities may exist.
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