In the present study the cerebellar afferents in the clawed toad Xenopus laevis have been analysed with the horseradish peroxidase (HRP) technique. In addition, data on the efferent connections of the cerebellum could be gathered, based on the phenomenon of anterograde transport of HRP. Cerebellar afferents in Xenopus laevis appear to arise mainly in the vestibular nuclear complex, in a primordial inferior olive and in the spinal cord. Both primary (arising in the ipsilateral vestibular ganglion) and secondary vestibulocerebellar projections were found. A distinct crossed olivocerebellar projection to the molecular layer of the cerebellum was found. Two spinocerebellar pathways are present in Xenopus laevis, as in other anurans, viz. an ipsilateral dorsal spinocerebellar tract, presumably arising in dorsal root ganglion cells, and a larger ventral pathway, bilaterally arising in the spinal gray matter. The latter tract mainly originates in the ventrolateral and ventromedial spinal fields. Furthermore, a secondary trigeminocerebellar projection arising in the descending trigeminal nucleus, a cerebellar projection arising in the dorsal column nucleus, a small projection arising in a possible primordium of the mammalian nucleus prepositus hypoglossi, a raphecerebellar projection, and a small cerebellar projection originating in the ipsilateral mesencephalic tegmentum were demonstrated. Cerebellar efferents in Xenopus laevis are mainly aimed at the vestibular nuclear complex. A distinct ipsilateral cerebellovestibular projection present throughout the vestibular nuclear complex presumably arises in Purkyn ĕ cells, a smaller contralateral projection in the cerebellar nucleus. In addition, a small primordial brachium conjunctivum, projecting to the red nucleus, was noted. The basic pattern of cerebellar connections as suggested for terrestrial vertebrates (ten Donkelaar and Bangma 1984) is also found in the permanently aquatic anuran Xenopus laevis.
