THE ORGAN OF HEARING. 



451 



DEVELOPMENT OF THE LABYRINTH. 

 The epithelial lining of the membranous labyrinth 

 is derived from the ectoderm and develops as a vesic- 

 ular invagination on each side of the epencephalon. 

 After being constricted off from the ectoderm this 

 vesicle develops a dorsomesial evagination, which 

 gradually grows larger and becomes the ductus endo- 



Fig. 310. Three transverse sections showing development of otic 

 vesicle of human embryo (Tourneux) : A, from embryo of 3 mm., showing 

 auditory pit; B, from embryo of 4 mm., showing the transformation of 

 the pit into the otic vesicle; C, from embryo of 6 mm., showing otic 

 vesicle detached from surface ectoderm, and presenting a posterior 

 diverticulum, the recessus vestibuli. 



lymphaticus. By means of folds and constrictions 

 the dorsal utriculus and ventral sacculus are formed, 

 and also the semicircular canals which connect with 

 the utriculus. The membranous cochlea or scala 

 media grows both in a longitudinal and a spiral 

 direction, retaining its connection with the sacculus 

 through the canalis reuniens. This complex mem- 



