. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. DEVELOPMENT OF EXTERNAL FORM 91. Fig. 55.—Floor of the Pharynx of Embryo B, 7 mm. Long. Ep, Epiglottis; Sp, sinus prsecervi- calis; t1, tuberculum impar; t2, posterior portions of the tongue; I, II, III, and IV, branchial arches. —(His.) chial" which is applied to them, though in the mammals they never have respiratory functions to perform, but, appearing, persist for a time and then either disappear or are applied to some entirely dif- ferent purpose. Indeed, in man they are never reall
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. DEVELOPMENT OF EXTERNAL FORM 91. Fig. 55.—Floor of the Pharynx of Embryo B, 7 mm. Long. Ep, Epiglottis; Sp, sinus prsecervi- calis; t1, tuberculum impar; t2, posterior portions of the tongue; I, II, III, and IV, branchial arches. —(His.) chial" which is applied to them, though in the mammals they never have respiratory functions to perform, but, appearing, persist for a time and then either disappear or are applied to some entirely dif- ferent purpose. Indeed, in man they are never really clefts but merely grooves, and corresponding to each groove in the ectoderm there is also one in the subjacent endoderm of what will eventually be the pharyn- geal region of the digestive tract, so that in the region of each cleft the ectoderm and endoderm are in close relation, being separated only by a very thin layer of mesoderm. In the intervals between successive clefts a more considerable amount of meso- derm is present (Fig. 55). In the human embryo four clefts and five branchial arches develop on each side of the body, the last arch lying posteriorly to the fourth cleft and not being very sharply denned along its posterior margin. As just stated, the clefts are normally merely grooves, and in later development either disappear or are converted into special structures. Occasionally, however, a cleft may persist and the thin membrane which forms its floor may become perforated so that an opening from the exterior into the pharynx occurs at the side of the neck, forming what is termed a branchial fistula. Such an abnormality is most frequently developed from the lower (ventral) part of the first cleft; normally this disappears, the upper portion of the cleft persisting, however, to form the external auditory meatus and tympanic cavity. A further stage in the differentiation of these clefts and arches is shown by the embryo represented in Fig. 56. The nape bend has no
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Keywords: ., bookautho, bookcentury1900, bookdecade1910, booksubjectembryology
