The development of the human body; a manual of human embryology . maxillary pro-cess, and the cleft separating them can be is due to a sinking inward of the region occupied bythe three posterior arches so that a triangular depression,the sinus prcecervicalis, is formed on each side of what willlater become the anterior part of the neck region. This iswell shown in an embryo (Br3) described by His whichmeasured mm. in length and of which the anterior por-tion is shown in Fig. 51. The anterior boundary of thesinus (ps) is formed by the posterior edge of the secondarch and
The development of the human body; a manual of human embryology . maxillary pro-cess, and the cleft separating them can be is due to a sinking inward of the region occupied bythe three posterior arches so that a triangular depression,the sinus prcecervicalis, is formed on each side of what willlater become the anterior part of the neck region. This iswell shown in an embryo (Br3) described by His whichmeasured mm. in length and of which the anterior por-tion is shown in Fig. 51. The anterior boundary of thesinus (ps) is formed by the posterior edge of the secondarch and its posterior boundary by the thoracic wall, and 102 THE DEVELOPMENT OF THE HUMAN BODY. in later stages these two boundaries gradually approachone another so as first of all to diminish the opening intothe sinus and later to completely obliterate it by fusingtogether, the sinus thus becoming converted into acompletely closed cavity whose floor is formed bythe ectoderm covering the three posterior arches andthe clefts separating these. This cavity eventually un-. 773 Fig. 51.—Head of Embryo of , Nasal pit; ps, precervical sinus.—(His.) dergoes degeneration, no traces of it occurring normallyin the adult, although certain cysts occasionally observedin the sides of the neck may represent persisting portionsof it. A somewhat similar process results in the closure of theventral portion of the first cleft,* a fold growing back- * See page 94, small type. THE DEVELOPMENT OF THE FACE. IO3 ward from the posterior edge of the first arch and fusingwith the ventral part of the anterior border of the secondarch. The upper part of the second cleft, however, persists,and, as already stated, forms the external auditory meatus,the pinna of the ear being developed from the adjacentparts of the first and second arches (Figs. 48 and 49). The region immediately in front of the first arch is
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
