Text III. The embryonal period of the oral cavity development.

A small depression (primary mouth) separated from the proximal gut by a pharyngeal membrane appears in a twelve-day embryo which is 2.5 millimetres long. By the end of the first month the mouth aperture is bounded by the mandibular processes below, by the maxillary processes above, and by the nasal processes above and along the midline.

In the second month the outer epithelial layer along the whole margin of the maxillary and mandibular processes thickens and protrudes into the oral cavity like a cord. Reproducing with the development of the embryo, the outer epithelial layer gradually shapes like an arch and separates into two plates, outer (bucco-labial) and inner (dental). The teeth form later from the inner plate.

From the fifth week the face grows faster and by the end of the second month acquires human features: the lateral nasal process develops into the wing of the nose, the lower portion of the median nasal process fuses with the maxillary processes to form the upper jaw. The median nasal process gives rise to the intermaxillary bone (premaxilla) and palatine processes which fuse with each other.

The oral cavity is separated from the nasal cavity in the third month when the hard and soft palates are completely formed. The palatine processes fuse by the end of the eight week, the fornix of the hard palate is already formed in the ninth week, and the fornix of the soft palate – in the twelfth week.

In case of the impaired embryonal development of the face in the first two months such malformations as hare-lip (labium leporinum) and cleft palate (faux lupinum) occur.

Text IV. Development of the jaws

The upper jaw (maxilla) forms from six bones nuclei that develop along the sides of the nasal capsule in the second month of gestation. The alveolar process separates from the bone germ; it contains the tooth germs and, growing further, forms the palatine processes and the frontal processes.

Five of the six nuclei which give rise to the maxilla fuse together already from the fourth month of intra-uterine life and form the foundation for the greater part of the alveolar process; the six nucleus which corresponds to the anterior portion of the alveolar process gives rise to separate bone, the premaxilla in which incisors develop.

Fusion of the premaxilla with the palatine and alveolar processes often begins already in the embryonal period. The suture formed at their union usually persists till adolescence, and traces of it often can be found in adults.

Each palatine bone develops from one ossification centre, arising posteriorly and somewhat medially of the maxillary germs in the eighth intra-uterine week. Growing upward and medially of the germs, each nucleus forms vertical and horizontal plates, which are a kind of accessory structures to the upper jaw; the horizontal plates contribute to the formation of the posterior portion of the hard and the lateral walls of the nasal cavity.

The lower jaw develops parallel with the development of the upper jaw as a paired bone from several ossification points which appear in the middle of the second embryonal month. At first the germ of the mandible looks like a groove enclosing the distal portion of the cartilage at the branchial arch, the middle portion of which reduces gradually and disappears. The anterior portion of the cartilage undergoes ossification and unites with the overlying bone, while the posterior portion turns into a ligament. Cartilaginous areas form in the proximal part of the developing mandible independent of Meckel’s cartilage. On ossification and fusion they form the condyloid process of the lower jaw which articulates with the temporal bone, and the coronoid process situated anteriorly. With the formation of the two processes a horizontally lying body and branches are distinguished in the lower jaw of the foetus. The alveolar process contains the tooth germs. By the time of birth both halves of the mandible are joined by a fibrous cartilage.