168 



THE DEVELOPMENT OF THE CHICK 



less during this period, and are destined to form the ei^ithelium 

 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 

 23atches 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 



/fiw. p.C/i 



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 



