The transition between the prosencephalon and the mesencephalon is marked by a relative narrowing at the dien-mesencephalic sulcus. There is also a narrowing between the mesencephalon and the rhombencephalon (isthmus). The transition from the rhombencephalon to the spinal cord (medulla spinalis) is in the region of the cervical flexure. At S14, following the growth of the two hemispheres and the appearance of a clear boundary (telodiencephalic or hemispheric sulcus), it is possible to distinguish the telencephalon and the diencephalon. There is no further segmentation of the mesencephalon. In the rhombencephalon, it is now possible to distinguish the metencephalon and the myelencephalon.