Notes: Summary We have reinvestigated the origin and genesis of the radial glia of the cerebellar cortex in the hamster using three astroglial markers, vimentin, GFAP, and S-100 protein antibodies. On embryonic day 12 (E12), before the emergence of the external granular layer, the cerebellar anlage is traversed from the ventricle to the pial surface by a primordial radial glial scaffold which is vimentin-positive, but GFAP and S-100 negative. With the formation of the external granular layer on E13, a few GFAP positive cells appear among the unstained external granular layer cells. First seen within the germinal trigone and caudalmost part of the external granular layer, they then develop rostrally, amongst the cells of the expanding external granular layer, proliferating adjacent to the basement membrane. Beginning on E15, cells that are positive for the S-100 protein also appear within the external granular layer and the molecular zone. In later stages, S-100 is strongly expressed in Golgi epithelial cells, so we have considered it to be a marker for these cells. By contrast, the primordial radial glial cells were not stained with this marker. On the day of birth (E16/PO) many S-100 positive cells also appear at intermediate levels between the EGL and the Purkinje cell plate. They are unipolar and bear a single radial process that is directed towards the pial surface. The caudorostral appearance of S-100-positive cells firstly in the external granular layer, then in the molecular zone and finally in the Purkinje cell plate is identical to the temporal sequence of development of these layers, and suggests that S-100-positive cells are at first integral constituents of the external granular layer, but later descend through the molecular zone, to colonize the Purkinje cell plate. Here they proliferate and ultimately differentiate into Golgi epithelial cells, their numerous short radial glial processes traversing the molecular zone and the external granular layer to fill the interstices between the primordial radial glial fibres. At birth, S-100-positive Golgi epithelial cells have progressively colonized the Purkinje cell plate from the germinal trigone rostrally, up to a region midway between primary fissure and anterior medullary velum and, between P2 and P3, the rostralmost part of the cerebellum has become populated. GFAP- and S-100-positive cells remain in the external granular layer up to the end of the first postnatal week. In the same interval, the number of Golgi epithelial cells and Bergmann glial fibres increases rapidly in the expanding cerebellar cortex. Our results suggest that the majority of the Golgi epithelial cells are not translocated, morphologically transformed primordial radial glial cells, but derive from the external granular layer, translocate into the Purkinje cell layer and differentiate into the secondary radial glial cells which intercalate with the basal processes of primordial radial glia. The latter are thus supplemented by the former, providing a radially organized substrate allowing granule cells produced in the secondary proliferative zone of the EGL to migrate through the molecular zone into the IGL.