THE ECTODERM AND ITS DERIVATIVES 



177 



later function as organs of equilibration in the adult. The junction of 

 each canal with the utricle causes the local enlargement of the 

 utricle to form an anipuHa. 



The saccule and the endolymphatic duct remain continuous. The 

 endolymphatic duct, however, has no auditory function but joins its 

 bilateral homologue to form the fused endolymphatic sacs which re- 

 main only as the vascular covering of the hindbrain. The saccule, 

 however, gives rise (at the 15 mm. stage) to two evaginations or sacs. 

 One of these is posterior and ventral, known as the cochlea or lagena, 

 and the other is slightly more dorsal (but posteriorly directed) and 

 is known as the pars basilaris (basilar chamber) of the ear. These 

 auditory structures never develop to the extent that they do in higher 

 vertebrates. However, in all vertebrates the lining epithelial walls of 

 the utricle (1), saccule (1), cochlea (3), and the various ampullae 

 (3) develop sensory patches (as numbered) which are connected 

 functionally with the eighth cranial or the auditory nerve. 



The sensory (auditory and equilibratory) parts of the ear are 

 therefore purely ectodermal in origin. However, the entire ear is en- 



Rhombencephalon 



Auditory 

 placode 



Cranial 

 nerve VII 



perior sinus 



Posterior semi- 

 circular canal 



Posterior ampulla 

 Pars basilaris 

 Lagena (cochlea) 

 Cranial nerve VIII 



Development of the auditory apparatus of the frog. (Redrawn after Krause.) 



