[Illustration: FIG. 34.—Diagrammatic representation of the development of the palatal processes and of the ethmo-vomerine plate, seen in vertical section. (After _Gegenbaur_.) _a._ Ethmo-vomerine plate. _b._ Palate processes. _c._ Tongue. _d._ Buccal cavity. _e._ Nasal cavity.] Each of these primary epiblastic pits is at a later date almost entirely cut off from its connection with the buccal cavity. The external and internal nasal processes of the fronto-nasal outgrowth unite below the anterior olfactory pits, and thus surround the anterior nasal apertures, and separate the nares from the mouth. The external nasal and superior maxillary processes are also freely amalgamated except along one small deep track, which remains patent to form the nasal duct and lachrymal passages; and probably the internal nasal and maxillary processes unite below the external nasal process to establish the continuity of the red margin of the upper lip. The union of all these various parts has been completed by the sixth to the tenth week of normal fœtal life; the external nasal and superior maxillary processes unite first, and by the sixth week are becoming closely approximated to the central portion of the fronto-nasal process, a time when the palatal processes are only indicated as slight ridges. By the ninth week the alveolus and upper lip are complete, and union of the palate is commencing from before backwards, being usually completed even to an indication of the uvula by the tenth week. Whilst the later stages of these developmental processes are in progress, points of _ossification_ have been appearing in many places to form the cranial and facial skeleton. A full knowledge of this subject is still unattained, but the researches of Goodsir, His, and others have thrown much light on hitherto dark passages. There are two main sources of origin of the bones of the skull, viz. from cartilage and from membrane, and it is important to appreciate the portions of the skull which originate from each of these sources respectively. The bones laid down primarily in cartilage are mainly those forming the base of the skull and their anterior prolongations. Thus about the fourth week of intra-uterine life the basis cranii consists of a cartilaginous mass surrounding the upper end of the notochord, and prolonged anteriorly around the pituitary fossa as two cartilaginous bars, the _trabeculæ cranii_, into the fronto-nasal process. From the anterior extremity of this the nasal bones and cartilages are developed, and from the under surface the ethmo-vomerine plate. The transformation of the primordial mesoblastic undifferentiated tissue into recognisable cartilage is occurring from about the fourth or fifth week until the eighth, when ossification at different centres is apparent. To the development of the intermaxilla we shall refer in detail later; suffice it to say here that the ossifying centres appear about the eighth week, and by the twelfth