. The development of the chick; an introduction to embryology . of the lens sac are at first of practically even thickness (28 s),but by the 35 s stage a great difference has arisen by the elonga-tion of the cells of the inner wall, which are destined to formlens fibers: the cells of the anterior (outer) wall elongate much 168 THE DEVELOPMEXT OF THE CHICK less during this period, and are tlestined to ioi-ni the e])ithehumof the lens (FIk- *)• Intermediate conditions are found aroundthe equator of the lens. The subsequent history is <;iven inchapter 1\. The Auditory Sac. At about the 12 s st
. The development of the chick; an introduction to embryology . of the lens sac are at first of practically even thickness (28 s),but by the 35 s stage a great difference has arisen by the elonga-tion of the cells of the inner wall, which are destined to formlens fibers: the cells of the anterior (outer) wall elongate much 168 THE DEVELOPMEXT OF THE CHICK less during this period, and are tlestined to ioi-ni the e])ithehumof the lens (FIk- *)• Intermediate conditions are found aroundthe equator of the lens. The subsequent history is <;iven inchapter 1\. The Auditory Sac. At about the 12 s stage the first evidenceof the auditory sacs is found in the form of a pair of circularpatches of thickened ectoderm situated on the dorsal surface ofthe 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. Fig. 97. — Transverse section through the eyes and heart of an embryo ofabout 35 s. The plane of the section will be readily understood by com-parison of Fig. Fis., Choroid fissure. D. C, Duct of Cuvior. Lg., Lung. pi. gr., Pleural groove. V. c, Posterior cardinal vein. Y. S., Yolk-sac. Other abbreviations as l)efore. is slightly depressed, and in the 16 s stage it forms a wide-openpit. At al)()ut the 20 s stage the mouth of the pit narrows slightly,and gradually closes (28-30 s), thus forming the auditory sac orvesicle (otocyst) (cf. Figs. 71. 73, S!), and 93). The method of dosui-e of the pit. which is of interest, mayreadily be ()l)S(M\-e(l in mounts of entiie embryos; at first thelil)s fold ()\-er most Iapidly from the anterior and posterior mar-gins; thus the mouth of the pit becomes elliptical with the longaxis vertical (stage of 22 somites) and extending from the apexnearly to the base. The ventral lip then begins to ascend (stageof 24 somites) and the closur
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