The cerebral hemispheres extend mainly in an anterior direction (anlage of the frontal lobe) over the telencephalon medium (lamina terminalis), upwards, and posteriorly (primordium of the parietal lobe). Later on during development, expansion also takes place from the rear, downwards and in an anterior direction (primordium of the temporal lobe). Whilst this growth takes place, the medial part of the base of the brain remains stable (anlage of the insula). The area of the insula can be thought of as an axis around which the hemispheres rotate (C-shape of the hemispheres and of several internal structures). The occipital lobe is formed later by expansion in a posterior direction. It does not undergo this 'rotational' movement, as demonstrated by the arrangement of the internal spaces (occipital horn of the lateral ventricle). Along the median plane, the medial surfaces of the hemispheres come together. The space between the two surfaces is called the longitudinal fissure. This is where the falx of the dura mater as well as the venous channels (superior and inferior sagittal sinus) develop. These structures drain blood via the anterior plexus and the v. capitis prima.