Rhombencephalon

With the development of the pontine flexure, the lateral part of the 4th ventricle is pushed further laterally. In this way, the 4th ventricle becomes rhomboid in shape. The thickened rhombic lips of the metencephalon are situated transversely and form the transverse cerebellar plate. The extra-ventricular cerebellum grows towards the outside and the intra-ventricular cerebellum enlarges into the ventricle. Towards the end of month 3, most of the cerebellum is extra-ventricular. From week 12 onwards, fissures lying perpendicular to the longitudinal axis of the brain can be distinguished. The first fissure to appear is the postero-lateral fissure, separating the flocculonodular lobe from the corpus cerebelli. Next to form are the fissura prima (between the anterior and posterior lobes) and the fissura secunda (between uvula and pyramis). Fissures as well as folia develop. They are also perpendicular to the longitudinal axis. Areas of the roof of the 4th ventricle, which initially are located in front of and behind the cerebellum, differentiate respectively into the superior and inferior medullary velum.