Concept of the development of the mammalian cerebellum based on physical rotation forces.

In pre-mammalian forms the cerebellum (rhombencephalon) has no rhombic lip and it develops in the median region between the two auricles above the front end of the fourth ventricle. In mammals, cells in the superior rhombic lip and dorsal part of the dorsal lamina of the metencephalon are said to proliferate to form two rounded swellings, the rudiments of the cerebellum. This assumption does not explain many peculiarities of the mammalian cerebellum, 11 of which have been described. The crux of the present hypothesis is that the mammalian cerebellar hemispheres and vermis develop from the median cerebellar element seen in the pre-mammalian forms and that the vermis does not develop as a confluence of the two hemispheric tissues.