. The development of the chick; an introduction to embryology . art of an embryo ofabout 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. Otherabbreviations as before. is slightly depressed, and in the 16 s stage it forms a wide-openpit. At about 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, 89, and 93). The method of clo
. The development of the chick; an introduction to embryology . art of an embryo ofabout 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. Otherabbreviations as before. is slightly depressed, and in the 16 s stage it forms a wide-openpit. At about 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, 89, and 93). The method of closure of the pit, which is of interest, mayreadily be observed in mounts of entire embryos; at first thelips fold over most rapidly 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 closure gradually proceeds towards the FROM TWELVE TO THIRTY-SIX SOMITES 169 ^- M. <;a rf§^ <*. apex, so that by the stage of 29 somites the opening is reducedto a minute eUipse situated on the external side of the dorsalmostportion of the otocyst (see Fig. 93). This portion of the otocystnow begins to form a small conicalelevation, and the final closure takesplace on the external side of thiselevation, which is destined toform the endolymphatic duct. Thelatter remains united to the epi-dermis at this point for a consid-erable period of time by a strandof cells which may preserve alumen up to 104 hours (Fig. 98).The final point of closure of the oto-cyst is thus very definitely placed,and it coincides with the middle ofthe endolymphatic duct, that is,with the junction of the later formedsaccus and ductus the Selachia this duct remainsin open communication with theexterior throughout life; the rela-tively long persistence of its con-nection with the epidermis in thech
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Keywords: ., bookauthorlilliefrankrattray187, bookcentury1900, bookdecade1910
