With the growth of the hemispheres, the originally transverse border between the telencephalon and diencephalon gradually becomes sagittally orientated. The caudal part of the medial surface of the hemispheres covers the lateral surface of the thalamus. The two parts of the brain are separated in the depth of the hemispheric sulcus by a thin layer of vascularized mesenchyme. The dorsal thalamus grows substantially in dorsal and lateral directions. As a consequence, the mesenchyme in the hemispheric sulcus disappears. The lateral part of the thalamus fuses together with the thin medial wall of the hemisphere. In this way, it ends up being covered by the ependyma of the lateral ventricle (lamina affixa) which is immediately ventral to the choroid fissure. The result of this fusion is that the lateral part of the thalamus comes into direct contact with the medial part of the striatum. The walls of the diencephalon and of the telencephalon are thus indirectly connected to one another. A pathway is created for the passage of cortical projection fibres and other fibre systems of the internal capsule. A transverse slit filled with pia mater remains on top of the 3rd ventricle only, between the telencephalon and the diencephalon: the transverse fissure.