Fig. 31A.—Head of fœtus at a somewhat later date (8 weeks) as seen from the front, showing the nasal and maxillary processes in close apposition, and the clefts between them diminishing in size.
Fig. 31B.—The same, seen from below, the mandibular process having been removed.
i.m. Central portion of fronto-nasal process, m.n.pr. Internal nasal process. pr.gl. Globular process, or the lower rounded extremity of the former. l.n.pr. External nasal process. m.x. Maxillary process. m.n. Mandibular process.
The most anterior of these depressions are the primary Olfactory pits which appear on either side on the lower surface of the wall of the anterior cerebral vesicle at a very early date. They are at first merely depressions surrounded by a raised margin; but subsequently they become pyriform by the extension of the lower end as a groove into the stomodæum. Each pit and groove is bounded laterally by thickened rounded outgrowths developed from a broad median fleshy protrusion from the same part of the cerebral vesicle, the fronto-nasal process; these are termed respectively the external and internal nasal processes, the latter being also named by His the globular processes ([Fig. 30]).
These Globular processes are separated in the median line by a groove which is subsequently obliterated by their amalgamation to form the central portion of the upper lip (“Philtrum” of German authors), and from their deep aspect the inner segments of the intermaxilla (endognathia). Above this groove is a central flattened median portion of the fronto-nasal process, from which subsequently the prominence of the nose is developed ([Figs. 31A, 32, and 33]), a result of the continued growth from its deeper aspect of the ethmo-vomerine plate.
The external nasal process forms the ala nasi and the soft parts in its immediate neighbourhood, extending downwards a little distance, but probably not so far as the red margin of the lip. From its deeper aspect the outer portion of the intermaxilla (mesognathion) is developed, and in it appears the germ of the lateral incisor. It also serves to separate the nasal pit from the second epiblastic intrusion which assists in the formation of the eye. The lower boundary of the naso-orbital fissure which passes from the primary ocular involution to the stomodæum is formed by the maxillary process, which is usually described as an upward extension of the mandibular process, but which probably arises separately as a pre-oral branchial outgrowth. From this is developed the whole of the superior maxilla, except that portion which is intermaxillary, and also the greater part of the cheek.
Figs. 32 and 33.—Later stages of development of fœtal head.