Embryonic rudiments of the cerebellum of 11 days old chick embryos were dissociated with trypsin; numerous spherules or globets of 100-300 µ diameter were re-aggregated after some hours; these were cultivated in vitro in a giratory shaker for a maximum of 21 days. Ultra thin sections for electron microscopy observation were made. Purkinje cells, granule cells and Golgi cells are recognizable. Various parts of these globets were found rich in synaptic knobs and other simple synaptic patterns as well some complex synaptic systems typical of normal cerebellum. Easily identifiable are cerebellar "glomeruli" formed of a central "rosette" synaptically joined with dendritic claws of the granule cells and "en marron" systems with the perikaryon of the Golgi cells. The complete absence of all extracerebellar afference is the proof that granule cells and Golgi cells are capable of synaptic connection with afferents of a different nature to those normally formed by mossy fibres and other fibres of extracerebellar origin. It is thus experimentally demonstrated that it is the recipient neuron which determines the synaptic pattern whatever the nature of the afference. It is also demonstrated that it is the activity of the efferent or recipient station which determines the synaptogenetic behaviour of nervous pathways.