In the 5 vesicle stage (6th week) the pontine flexure develops in the rhombencephalon which divides it into a metencephalon and myelencephalon. The metencephalon is more cranial and forms the pons and cerebellum; the myelencephalon becomes the medulla. The presumptive site of the cerebellum is seen as the rhombic lips at the cranial edge of the thin roof of the 4th ventricle. A depression develops in the prosencephalon which defines the telencephalon from the diencephalon. Subsequent growth of the telencephalon will cause it to expand dorsally, caudally, laterally, and inferiorly.

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