A PHASEOLUS LECTIN ANTEROGRADE TRACING STUDY OF THE TECTOROTUNDAL PROJECTIONS IN THE DOMESTIC CHICK

The projection from the chick optic tectum to the nucleus rotundus was investigated by iontophoresis of the anterograde tracer Phaseolus vul garis lectin (PHA-L) into the optic tectum and by Golgi impregnation. PHA-L labelled fibres were observed within the ipsilateral nucleus rot undus and to course through the supraoptic decussation to the contrala teral nucleus rotundus. They appeared similar to fibres evident in Gol gi impregnated material. Before reaching the nucleus rotundus, PHA-L l abelled fibres gave off short terminal branches in the caudoventral th alamic nucleus. Within the ipsilateral nucleus rotundus, the labelled fibres formed an extensive fibre net with a dense core and a less dens e periphery. PHA-L labelled fibres in the contralateral nucleus rotund us exhibited a qualitatively similar but sparser distribution. There a ppeared to be a topographic representation of the optic tectum within the nucleus rotundus. Electron microscopy revealed that PHA-L labelled tectal fibres established asymmetric synapses with principal neurons in the nucleus rotundus.