The development of the cerebellum involves a set of coordinated cell movements and two separate proliferation zones: the ventricular zone and the external granule cell layer (EGL), a rhombic-lip-derived progenitor pool. The EGL appears segregated during early cerebellum formation and produces only granule cells. Cerebellar granule cells (CGC) are the most abundant neurons in the brain, about 1 x 10^11 in humans. Their axons run as parallel fibres along the coronal axis, and the one-dimensional spread of excitation that results from this arrangement is a key assumption in theories of cerebellar function. CGC receive inhibitory synaptic input from Golgi cells, which are mediated by gamma-aminobutyric acid (GABA). During both in vivo and in vitro development, CGC depend on the activity of the NMDA glutamate receptor subtype for survival and full differentiation. Cultured CGC are widely used as a model system for studying neuronal apoptosis.