Development of the oculomotor and trochlear nuclei in the Xenopus toad.

The development of the oculomotor and trochlear nuclei (nIII and nIV) was studied with the horseradish peroxidase (HRP) and the cobalt labelling techniques in Xenopus laevis tadpoles. The earliest labelling of the oculomotor neuroblasts was observed at stage 32. The ipsi- and contralateral nuclei were found in two distinct groups on either side of the brainstem and the oculomotor commissure formed by crossing axons was present at this early stage. The fusion of the two nuclei began at the late larval stage when the axonal outgrowth had been presumably completed. The trochlear neuroblasts could be first labelled at stage 39 when the position of the nucleus and axonal pathway was similar to the adult form.