. The development of the chick : an introduction to embryology. Embryology; Chickens -- Embryos. 168 THE DEVELOPMENT OF THE CHICK less during this period, and are destined to form the epitheUum of the lens (Fig. 97). Intermediate conditions are found around the equator of the lens. The subsequent history is given in chapter IX. The Auditory Sac. At about the 12 s stage the first evidence of the auditory sacs is found in the form of a pair of circular patches of thickened ectoderm situated on the dorsal surface of the head opposite to the ninth, tenth, and eleventh neuromeres, and thus a short
. The development of the chick : an introduction to embryology. Embryology; Chickens -- Embryos. 168 THE DEVELOPMENT OF THE CHICK less during this period, and are destined to form the epitheUum of the lens (Fig. 97). Intermediate conditions are found around the equator of the lens. The subsequent history is given in chapter IX. The Auditory Sac. At about the 12 s stage the first evidence of the auditory sacs is found in the form of a pair of circular patches of thickened ectoderm situated on the dorsal surface of the head opposite to the ninth, tenth, and eleventh neuromeres, and thus a short distance in front of the first mesoblastic somite; it lies between the rudiments of the acustico-facialial and glosso- pharyngeal ganglia. In the 14 s stage the auditory epithelium le/is. aCA. Lens . Fig. 97. — Transverse section through the eyes and heart of an embryo of about 35 s. The plane of the section will be readily understood by com- parison of Fig. 117. ch. Fis., Choroid fissure. D. C, Duct of Cuvier. Lg., Lung. pi. gr., Pleural groove. V. c, Posterior cardinal vein. Y. S., Yolk-sac. Other abbreviations as before. is slightly depressed, and in the 16 s stage it forms a wide-open pit. At about the 20 s stage the mouth of the pit narrows slightly, and gradually closes (28-30 s), thus forming the auditory sac or vesicle (otocyst) (cf. Figs. 71, 73, 89, and 93). The method of closure of the pit, which is of interest, may readily be observed in mounts of entire embryos; at first the lips fold over most rapidly from the anterior and posterior mar- gins; thus the mouth of the pit becomes elliptical with the long axis vertical (stage of 22 somites) and extending from the apex nearly to the base. The ventral lip then begins to ascend (stage of 24 somites) and the closure gradually proceeds towards the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations m
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Keywords: ., bookauthorlilliefrankrat, bookcentury1900, booksubjectembryology
