The cartilage bones of the occipital segments are derived from the parachordals of the embryonic skull, those of the parietal and frontal segments from the trabeculae.
In front of the presphenoid the basicranial axis is continued by the mesethmoid.
(2) The sense capsules.
These enclose and protect the special sense organs.
(a) Auditory capsule.
The basisphenoid is always continuous with the basi-occipital, but the alisphenoid is not continuous with the exoccipital as the periotic or auditory capsule is interposed between them. Each periotic capsule has three principal ossifications; an anterior bone, the pro-otic, a posterior bone, the opisthotic, and a superior bone, the epi-otic.
These bones may severally unite, or instead of uniting with one another they may unite with the neighbouring bones. Thus the epi-otic often unites with the supra-occipital, and the opisthotic with the exoccipital.
Two other bones developed in the walls of the auditory capsule are sometimes added, as in Teleosteans; these are the pterotic and sphenotic.
(b) Optic capsule.
The eye is frequently enclosed in a cartilaginous sclerotic capsule, and in this a number of scale-like bones are often developed.