The expansion of the hemispheres can be described as a 'rotation' around a transverse axis passing through the primordium of the insula and the neurons concentrated in the ventricular eminence. Some internal structures developing during this period are involved in this process and take a more or less noticeable C-shape. The structures most influenced are the hippocampal formation and the fornix, the cingulate gyrus and its prolongation into the parahippocampal gyrus, the caudate nucleus, the stria terminalis, the lateral ventricle. The corpus callosum has a less marked C-shape.
An invagination in the lateral ventricle is formed immediately above the diencephalon on the medial wall of the hemispheres: the choroid fissure. The epithelium of the pallium thins out in this region. Parts of the surrounding pia mater enter the ventricle. Blood vessels and mesenchymal cells from the connective tissue between the hemispheres penetrate into the core of these lobules and villi to form the choroid plexus of the lateral ventricle. The plexus takes part in the 'rotation' of the hemispheres. Due to this, it can be seen in the frontal horn, the central part and the temporal horn of the lateral ventricle. There is no plexus in the occipital horn because the occipital lobe develops later and is not part of the rotation.