Optic tectum in congenitally monophthalmic fishes and chicks.

Morphometry of optic tectum layers was performed in fishes and birds that were congenitally monophthalmic (or had one very small or malformed eye). The optic chiasm was studied with regard to possible anomalies of the fiber distribution. In two very young fish larvae with only one ocular rudiment, no differences were found between the two tecta. In fishes and chicks with one normal and one eye of reduced size, the volume of superficial tectal layers contralateral to the small eye was significantly more affected (reduction by 40 to 70%) than that of the deep layers (reduction by 10 to 30%). Particularly pronounced differences between the superficial tectal layers of both sides were found in a fish where a thin optic nerve, originating in an unusually small eye, projected to the ipsilateral side of the brain. It is concluded that (1) the development of superficial tectal layers depends more strongly on optic innervation than that of deep layers; (2) obvious differences between tecta with and without optic innervation develop rather late in ontogeny; (3) there seems to be no measurable difference between effects of a poor (less than 25%) innervation of the tectum and a complete lack of this projection.