In the parachordal region, the unsegmented paraxial mesoderm forms the plate-like parachordal cartilage (cartilago parachordalis) or basal plate, which contains the rostral part of the notochord. The occipital sclerotomes are just caudal to the basal plate. They lose their segmentation and interconnect with it. At S18-19, the basal plate and the fused rudimentary segments of the occipital sclerotomes (separated by the fascicles of the hypoglossal nerve) form the basilar part of the occipital bone (basi-occipital). Later on chondrification extends from the parachordal centre dorsally into the occipital region as the exoccipital part. It surrounds the spinal cord and the medulla oblongata and forms the foramen magnum. The dorsal part of this clasp surrounding the medulla is called the occipital tectum (supra-occipital).
The prechordal region includes both the hypophysial and interorbitonasal regions. The paired hypophysial cartilages develop around the pituitary gland, whilst the paired trabecular cartilages develop rostrally to it. These four chondrification centres come together and form the body of the sphenoid (basisphenoid, os sphenoidale) and the ethmoid (os ethmoidale). They also connect to the basilar part of the occipital bone. Thus, a cartilaginous basal plate is formed which extends from the nasal area to the foramen magnum.