[Illustration: FIG. 29.—Head of fœtus, of about 5 weeks, from ventral aspect (after His), showing the primitive stomodæum bounded above by the undivided fronto-nasal process, laterally by the maxillary, and below by the still separate mandibular processes. The quinqueradiate appearance is well represented. (_Sutton._)] [Illustration: FIG. 30.—Head of fœtus from ventral aspect of a little later date (6-7 weeks). The mandibular processes have now united; the orbito-nasal fissure has come in contact with the ocular vesicle, and the fronto-nasal process has developed into external and internal nasal (or globular) processes around the nasal fossæ (after His). (_Sutton._)] At the same time that this mandibular arch is being developed, other changes are occurring around the upper part of the stomodæum, viz. the shutting off of sacs lined with epiblast to assist in the formation of the organs of special sense, and the outgrowth between them of fleshy processes which by their later amalgamation form the facial skeleton and coverings. Three of these involutions of epiblast occur, two communicating more or less with the stomodæum, viz. the nasal and ocular; whilst the third, or auditory, is separate. Expansions from the cerebral vesicles meet them, and by further changes, unnecessary to particularize here, the organs of special sense are elaborated. [Illustration: 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).