Enlargement of the frontal, parietal and temporal parts of the hemispheres places them on a different level with respect to the insula. It also prompts formation of a groove between the fronto-parietal and temporal portions, which will become the lateral sulcus. Neighbouring parts of the frontal, parietal and temporal lobes grow little by little over the area of the insula (frontal, parietal and temporal opercula). The walls of the hemispheres are initially smooth. Following substantial growth of the cerebral cortex, the wall begins to form furrows (sulci) and convolutions (gyri). During month 5, the cingulate sulcus forms after the lateral sulcus. It is only during month 6 that the central sulcus becomes visible. The primary sulci are those which are recognizable in every brain. They appear before the end of month 8. The secondary and tertiary sulci appear later and vary in shape from one person to another. During growth, the hemispheres of the brain initially grow over the diencephalon, then also over the mesencephalon. They finally border the cerebellum, from which they are separated by the tentorium cerebelli. Towards the end of the embryonic period, the olfactory bulb forms at the base of the frontal lobe as an evagination of the lateral ventricle. It makes contact with the olfactory epithelium in the roof of the nasal cavity through the fila olfactoria.