The bones of the roof of the skull and of the face are chiefly derived from membrane, those of the base of the skull being laid down in cartilage.

At the cephalic extremity of the notochord is a mass of uncleft mesoblast, called the investing mass, corresponding to that from which the vertebrae are developed.

From this arises two prolongations, which diverge and then unite again, leaving an interval; and the united portion becomes once more divided into two processes, the trabecules cranii.

Basis cranii of a chick, sixth day. {Huxley).

Fig. 316. Basis cranii of a chick, sixth day. {Huxley).

1. Chorda dorsalis.

2. Basal cartilage.

3. Trabecular.

4. Pituitary space.

5. Internal ear.

Longitudinal section through the head of an embryo of four weeks.

Fig. 317. Longitudinal section through the head of an embryo of four weeks. (K'ol-liker).

v. Cavity of cerebral hemisphere. Optic vesicle.

z. Cavity of third ventricle.

m. Cavity of mid-brain.

h. Cerebellum.

n. Medulla.

o. Auditory depression.

t. Basis cranii.

t'. Tentorium.

p. Pituitary body.

The mesoblast behind the interval receives the name of the occipito-sphenoid portion; the interval is the rudiment of the sella tunica, which is occupied by the pituitary body. The part of the mesoblast in front of this is called the spheno-ethmoidal portion.

From the occipito-sphenoidal portion are developed the basi-occipital and the posterior part of the sphenoid. At the sides of the medulla oblongata processes are sent up, which unite round it and form the foramen magnum. Laterally the mesoblast envelops the auditory vesicles and forms the side portions of the occipital bone.

In the cartilaginous antecedent of the temporal bone there are three centres of ossification - the epiotic, which develops the mastoid process; the probtic, which is in the region of the superior semicircular canal; and the opisthotic, which is at the cochlea.

The spheno-ethmoidal portion develops the anterior part of the sphenoid together with the ethmoid bones and the cartilage of the septum of the nose; the first, arising from the back part, is developed from membrane. The trabecule are carried forward, and bending down at the nasal depression form the lateral nasal cartilages and the anterior part of the septal cartilage.

Different stages of the development of the head and face of a human embryo.

Fig. 318. Different stages of the development of the head and face of a human embryo.

A. Embryo of four weeks. (Allen Thomson.) B. Embryo of six weeks. (Ecker).

C. Embryo of nine weeks. (Allen Thomson).

a. Auditory vesicle. 1. Lower jaw. 1'. First pharyngeal cleft.

The face is developed in connection with ridges known as the visceral'folds or arches, between which are a number of clefts, the visceral clefts.

The eyes and the openings of the nose are in the face; while the ear arises at the side of the face, in connection with one of the visceral clefts.

The nasal depressions or pits appear in the wall of the head, covering the anterior part of the brain.

Just above the first visceral arch or fold is the depression which ultimately becomes the buccal cavity, and unites with the alimentary tract to form the mouth.

The first fold is called the mandibular; this gives off at either end a process which grows upward and inward, forming the rudiment of the superior maxillary bone and side of the face.

Between these is a median process, the fronto-nasal, which gives off, on the inner sides of the nasal grooves, projections which form the inner nasal processes; these unite with the superior maxillary processes to close in the nostril and form the lip.

The outer nasal process is a thickening on the outer side of the nasal depression, which running down toward the superior maxillary process, forms eventually the lachrymal duct.

Vertical section of the head of an embryo of a rabbit.

Fig. 319. Vertical section of the head of an embryo of a rabbit. (Mihalkovics).

In A. there is no connection between the buccal cavity and the fore-gut. In B. the connection is established.

m. Epiblast of neural canal, h. Heart, c. Cavity of fore-brain, mc. Cavity of mid-brain. mo. Cavity of medulla, sp.o. Sphenooccipital parts of the basis cranii. sp.e. Sphenoethmoidal part of the basis cranii. be. Part of basis cranii which receives the pituitary body. am. Amnion, py. Part of heart cavity going to form the pituitary body, i.f. Fore-gut. sh. Notochord. if. Infundibulum.

The mandibular arch forms the lower jaw, and between this and the superior maxillary process the buccal cavity is developed chiefly by the outgrowth of the surrounding tissues; the epiblast lining this becomes thinned away, and the subjacent mesoblast and hypoblast disappear; the buccal cavity is made continuous with that of the alimentary canal.

The cavities of the nasal depressions at first open freely into the buccal cavity by means of the nasal grooves; after a time processes arise from the superior maxillae, and, growing inward, finally meet one another in the middle line, to form a broad plate of tissue intervening between the nasal cavity above and the buccal cavity below. This plate is first completed in front and then gradually closes toward the back of the buccal cavity, where the communication between the nose and the pharynx is left.

Imperfect development of these parts gives rise to the common congenital deformities, cleft-palate and hare-lip.

The first cleft is the hyo-mandibular; it forms the tympano-Eustachian cavity, which becomes separated from the surface by the closure of its outer end by the growth of the membrana tympani, the external auditory meatus and ear being formed by an outgrowth of the tissue surrounding the tympanic membrane.

The mandibular arch contains, close to its connection with the superior maxillary process, a rod of cartilage, called Meckel's cartilage. This becomes partly converted into the malleus, partly into the internal lateral ligaments of the temporo-maxil-lary articulation.

The second, or hyoid arch, gives origin to the incus, the stylohyoid process and ligament, and the lesser wings of the hyoid bone.

From the third arch arise the body and greater wings of the hyoid bone and the thyroid cartilage.