Immature cerebellum, light micrograph. This is the external granular layer. Cerebellar granule cells are also the most numerous neurons in the brain (50 million in humans). Granule cell precursors are produced and migrate away from the rhombic lip to populate the external granular layer. The external granular layer is a secondary germinal zone that produces the millions of granule cell progenitors that will later differentiate and then migrate into the core of the cerebellum using Bergmann glia astrocytes as a guide.