The dorsolateral parts of the alar plates bend medially and form the rhombic lips (Fig. 19.18). In the caudal portion of the metencephalon, the rhombic lips are widely separated, but immediately below the mesencephalon they approach each other in the midline (Fig. 19.20). As a result of a further deepening of the pontine flexure, the rhombic lips compress cephalocaudally and form the cerebellar plate (Fig. 19.20). In a 12-week embryo, this plate shows a small midline portion, the vermis, and two lateral portions, the hemispheres. A transverse fissure soon separates the nodule from the vermis and the lateral flocculus from the hemispheres (Fig. 19.20B). This flocculonodular lobe is phylogenetically the most primitive part of the cerebellum.

Initially, the cerebellar plate consists of neuroepithelial, mantle, and marginal layers (Fig. 19.21 A). During further development, a number of cells formed by the neuroepithelium migrate to the surface of the cerebellum to form the external granular layer. Cells of this layer retain their ability to divide and form a proliferative zone on the surface of the cerebellum (Fig. 19.21, B and C).

In the sixth month of development, the external granular layer gives rise to various cell types. These cells migrate toward the differentiating Purkinje cells (Fig. 19.22) and give rise to granule cells. Basket and stellate cells are produced by proliferating cells in the cerebellar white matter. The cortex of the cerebellum, consisting of Purkinje cells, Golgi II neurons, and neurons produced by the external granular layer, reaches its definitive size after birth (Fig. 19.22B). The deep cerebellar nuclei, such as the dentate nucleus, reach their final position before birth (Fig. 19.21 D).

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