THE LATER DEVELOPMENT OF THE CHICK 323 



endolymphatic sac. This finally extends above the central 

 nervous system, and lies embedded in the mesenchyme along 

 the dorso-lateral surface of the myelencephalon. 



The superior chamber of the otocyst gives rise to the semi- 

 circular canals and the utricle. The semicircular canals are in- 

 dicated the fifth day, as three slight grooves in the wall of the 

 superior chamber, approximately in the relative positions of the 

 fully developed canals, save that the posterior canal is at first 

 oblique to the other two. These grooves deepen and their 

 margins meet converting them into canals, which remain open 

 into the chamber at their extremities. The canals push out 

 from the surface of the otocyst carrying a thin sheet of its 

 wall which becomes perforated between the body of the superior 

 chamber and the canals, leaving them free, except at their 

 attached ends. The cavity of the superior chamber remaining, 

 after the formation of the semicircular canals, is the utricle; this 

 receives also the opening of the endolymphatic duct, and 

 ventrally it opens into the inferior chamber of the otocyst. 

 The ampullce of the canals appear as dilations very early 

 (seventh day). 



The inferior chamber of the otocyst is the seat of origin of 

 the saccule and the lagena or cochlea. The saccule appears 

 (seventh day) as a swelling on the inner or medial wall of the 

 extreme upper (dorsal) end of the inferior chamber. The 

 ventral end of this chamber pushes downward as the rudiment 

 of the lagena or cochlea, while the intermediate region remains 

 as the cochlear duct. The lagena grows out for some distance, 

 turning inward (medially) at its tip, forming as a whole a 

 hook-shaped structure (Fig. 129, B). 



The epithelial lining of the otocyst has meanwhile become 

 thinner, except in certain patches which mark the location of 

 the maculce, cristce, and papillce of the fully formed ear. In 

 these regions the epithelium assumes the typical character- 

 istics of the acustic epithelium, and into each patch there 

 extend nerve fibers (axons) from the cells of the VIII nerve 

 ganglion, which is intimately fused with the antero-ventral 

 face of the otocyst. 



