168 THE DEVELOPMENT OF THE CHICK 
less during this period, and are destined to form the epithelium 
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 12s 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 14s stage the auditory epithelium 
la 4S, 
(@) 
Lens . =G 
Fig. 97. — Transverse section through the eyes and heart of an embryo of 
about 35s. The plane of the section will be readily understood by com- 
parison of Fig. 117. 
ch. Fis., Choroid fissure. D. C., Duet of Cuvier. Lg., Lung. pl. gr., 
Pleural groove. V. ¢., Posterior cardinal vein. Y. 8., Yolk-sae. Other 
abbreviations as before. 
is slightly depressed, and in the 16s stage it forms a wide-open 
pit. At about the 20s stage the mouth of the pit narrows slightly, 
and gradually closes (28-30 s), thus forming the auditory sac or 
vesicle (otocyst) (ef. 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 
