The development of connections between the isthmic nucleus and the tectum in Xenopus and Limnodynastes tadpoles.

In adult Anura reciprocal connections exist between the isthmic nucleus and ipsilateral tectum. These pathways, with a projection from the isthmic nucleus to the opposite tectum, constitute the binocular visual connections. Isthmic nuclei were examined histologically after injection of horseradish peroxidase into the tectum of Xenopus laevis and Limnodynastes dorsalis from midlarval stages to the completion of metamorphosis and in adults. The isthmic nuclei were present throughout, their cell number reaching the adult complement by metamorphic climax. At all stages each isthmic nucleus projected to both tecta. Late onset of electrical activity in the intertectal pathway cannot therefore be explained by the time at which isthmic fibres reach the tecta.