The cartilaginous base of the cranium is now formed, but the vault is membranous. While the base has been developing the two anterior visceral arches have been also forming and have gained an attachment to the cranium, but the formation and fate of these is recorded in the article SKELETON (Visceral). About the sixth week of foetal life ossification begins at different points in the membranous vault of the skull. In this way the frontal, parietal, supra-occipital, and a little later the squamous part of the tern poral bones are formed. About the eighth week, too, the lachry mal, nasal and vomer appear in the membrane lying superficial to different parts of the olfactory capsule. All these are dermal bones, comparable to the deeper parts of the scales of fishes, and developed in the mesenchyme lying deep to and in contact with the ectoderm. It is therefore necessary to think of the primitive skull as a three-layered structure, the deepest layer persisting as the dura mater, the middle forming the chondro-cranium, which ossifies to form the base, and a superficial layer close to the skin or mucous membrane (ectoderm), from which the bones of the vault and superficial parts of the olfactory capsules are derived. At the four angles of the parietal, ossification is checked for some time to form fontanelles, of which the bregma is the most impor tant, and at each of these points, as well as elsewhere in the sutures, accessory centres of ossification may occur to form W ormian bones.
Along the middle line of the base of the skull the same progress of ossification from behind forward is seen that was noticed in the process of chondrification. Bilateral centres for the basi occipital appear about the sixth week, for the basisphenoid in the eighth, and for the presphenoid in the tenth, while the lateral mass of the ethmoid does not ossify till the fifth month and the meseth moid not until the first year of extrauterine life. In the lateral part of the base the exoccipitals and alisphenoids begin to ossify about the eighth week and the presphenoids about the tenth. In connection with the alisphenoid there is a small extra centre of morphological interest only, which forms a little tongue-shaped process called the lingula, projecting back into the middle lacer ated foramen and apparently corresponding to the sphenotic bone of lower vertebrates.
The auditory or periotic capsule, like the olfactory, is late in ossifying; it has four centres (pro-otic, epiotic, opisthotic and pterotic) which do not come until the fifth month.
Some parts of the chondro-cranium do not ossify at all; this is the case in the anterior part of the mesethmoid, which remains as the septal cartilage of the nose, while, as has been already pointed out, a buffer of cartilage persists between the basioccipital and basisphenoid until the twentieth year of life.
From what has been said it is evident, and it will be still more evident if the article SKELETON (Visceral) be looked at, that some of the bones of the adult skull are compounded of various contri butions from the different elements which make up the adult cranium. These, recapitulated, are (I) the dura mater or endo
cranium, which in man does not ossify except perhaps in the crista galli. (2) The chondro-cranium or mesocranium. (3) The superficial part of the mesenchyme (ectocranium) from which dermal bones are formed. (4) The olfactory and auditory sense capsules. (5) The visceral arches. (6) Some fused vertebrae posteriorly.
The occipital bone (fig. 7) for example, has the basioccipital, exoccipital and basal part of the supra-occipital derived from the chondro-cranium and fused vertebrae, while the vault part of the supra-occipital has four dermal centres of ossification correspond ing to the interparietal and preinterparietal bones of lower mam mals. The bone of Kerkring is an abnormality, the meaning of which is not understood.
The temporal (fig. 8) is also a very composite bone; in it the petro-mastoid portion represents the auditory sense capsule; the tabular external auditory meatus is formed by the outgrowth of the tympanic ring which is probably part of the first visceral arch (see SKELETON, Visceral) ; the squamo-zygomatic part is a dermal bone, while the styloid process is a part of the second visceral arch.
The mastoid process is not present at birth, but appears about the second year and becomes pneumatic about puberty. From what has been seen of the skull bones in this necessarily concen trated and abridged account, it is obvious that they do not cor respond to the traces of segmentation as indicated by the cranial nerves, and for this and other reasons the "vertebrate theory of the skull" is no longer believed in.
For further details and references see The Development of the Human Body, J. P. McMurrich (London, 1923) and other stand ard anatomical textbooks previously named.
In this section only those parts of the skull which form the covering for the brain and the capsules for the olfactory and auditory apparatus are considered. Those parts of the face and jaws which are developed in connection with the visceral arches are dealt with in the article SKELETON (Visceral). In the Acrania (Amphioxus) the enlarged anterior end of the nerve cord is merely surrounded by fibrous tissue continuous with the sheath of the rest of the nerve cord; there is therefore, in a sense, no true cranium.
In the Cyclostomata (hags and lampreys) a cartilaginous cra nium is developed, the anterior part of which forms an unpaired olfactory capsule connected with the rest of the cranium by fibrous tissue only. In the floor, just in front of the anterior end of the notochord, an aperture, the basi-cranial fontanelle, remains un chondrified for the passage of the pituitary diverticulum into the skull.