322 OUTLINES OF CHORDATE DEVELOPMENT 



able sac, the auditory sac or otocyst, connected with the surface 

 by a narrow canal. The otocyst very early becomes vertically 

 elongated and soon shows internally an oblique ridge on its 

 inner or medial surface, indicating its division into an upper 

 and inner part, and a lower and outer part, known respectively 

 as the superior and inferior chambers. The superior chamber 

 is extended vertically as a short tube with which the canal 

 leading to the surface is connected. This tube is the rudiment 

 of the endolymphatic duct (Fig. 129, A). On account of the 

 dorsal expansion of the superior chamber along the outer side 



a 



B 



FIG. 129. Two stages in the development of the auditory organ of the chick. 

 A. Hemisected model of the auditory sac (otocyst) just before its separation 

 from the superficial ectoderm of the head. After Krause. B. Median view 

 of a model of the left membranous labyrinth of an embryo of seven days and seven- 

 teen hours. After Rothig and Brugsch. a. Anterior vertical semicircular 

 canal; aa, ampulla ^of anterior vertical semicircular canal; ap, ampulla of poste- 

 rior vertical semicircular canal; d, ductus endolymphaticus; e, superficial ecto- 

 derm of head; I, lagena (cochlea); p, rudiment of posterior vertical semicircular 

 canal; s, rudiment of saccule; u, utricle; x, connection between auditory sac and 

 superficial ectoderm. 



of the endolymphatic duct, the latter appears to open into 

 the inner side rather than into the apex of the superior chamber. 

 The narrow canal of the duct becomes closed during the fifth 

 day, and the otocyst soon thereafter loses all connection with 

 the surface ectoderm. 



We may now mention the more important steps in the 

 development of each of these three primary regions of the 

 otocyst. The endolymphatic duct grows dorsally during the 

 seventh and eighth days, and its extremity dilates forming the 



