The development of the prosencephalon of the mouse has been studied from the 8th to the 13th day post-conception, and some details of mammalian prosencephalic morphogenesis have thus been elucidated. The temporal pattern of development has been analysed in serial semithin coronal sections, and the prosencephalon reconstructed at different ages. The morphological differentiation of the cerebral hemispheres is dependent on a combination of cell proliferation and cell death. Necrosis thins the prosencephalic walls in certain regions (lamina terminalis and "choroid plate"). It is around the lamina terminalis, the choroid plate, and the zone of fusion of the walls of the dorsal and ventral telodiencephalic sulci that the prosencephalic vesicle develops. Thus, the olfactory bulbs are formed rostrally and the lateral ventricles laterally. The fusion between the telencephalic and diencephalic walls occurs at the ependymal surfaces of the dorsal and ventral telodiencephalic sulci, and not at their pial surfaces. The limits between the lateral ventricles and the 3rd ventricle are constituted rostrally by the lamina terminalis, dorsally and ventrally by a region where the corpus striatum mediale and the "choroid plate" meet, and caudalli by the fusion of the ependymal surfaces of the dorsal and ventral telodiencephalic sulci.