Sense capsules. Laterally to the prechordal region, other cartilaginous centres develop to form the orbit. At S20, the cartilaginous centre located most rostrally forms the orbital wing or orbitosphenoid (primordium of the lesser wing of the sphenoid), which surrounds the optic nerve. The caudal cartilaginous centre forms the temporal wing, from which the greater wing of the spenoid develops at S20.
The cartilaginous nasal capsules develop around the olfactory pit. The two nasal capsules and the trabecular cartilage fuse together.
By S17, the mesoderm surrounding the otic vesicle forms the otic capsule and undergoes chondrification. The otic capsule is located laterally to the basal plate (parachordal region) and later fuses together with it. During the process of fusion, the passages through which the jugular vein and the glossopharyngeal, vagus and accessory nerves leave the skull (jugular foramen) are left open.The otic capsule remains open where the facial and vestibulo-cochlear nerves pass through (internal acoustic meatus).
Ossification of the cartilaginous cranial base begins at week 9. However, much of the connecting zones between the various primordial bones (synchondroses) remain cartilaginous for quite a long time. This enables the base of the skull to adapt to the growth of the brain. In the newborn, some synchondroses have already ossified and become synostoses. The connection between the sphenoid and basilar part of the occipital bone (spheno-occipital synchondrosis) remains cartilaginous until puberty.