160 THE THIRD DAY. [CHAP. 



vestibuli, and into a bony cochlea; but the junction 

 between the cochlea and the bony vestibule is much 

 wider than the membranous canalis reuniens. 



The cavity of the osseous cochlea is partially divided 

 lengthways by the ductus cochlearis into a scala tym- 

 pani and a scala vestibuli, which do not however extend 

 to the lagena. 



The auditory nerve, piercing the osseous labyrinth 

 in various points, is distributed in the walls of the mem- 

 branous labyrinth. 



All these complicated structures are derived from 

 the simple primary otic vesicle and the surrounding 

 mesoblast by changes in its form and differentiation of 

 its walls. All the epiblast of the vesicle goes to form 

 the epithelium of the membranous labyrinth, whose 

 cavity, filled with endolymph, represents the original 

 cavity which was first open to the surface but subse- 

 quently covered in. It gradually attains its curiously 

 twisted form by a series of peculiar processes of unequal 

 growth in the, at first, simple walls of the vesicle. The 

 corium of the membranous labyrinth, and all the tissues 

 of the osseous labyrinth, are developed out of the meso- 

 blastic investment of the vesicle. The space between 

 the osseous and membranous labyrinths, inchiding the 

 scala vestibuli and scala tympani, may be regarded as 

 essentially a series of lymphatic cavities hollowed out 

 in the mesoblast. 



It will be seen then that the ear, while resembling 

 the eye in so far as the peculiar structures in which the 

 sensory nerve in each case terminates are formed of 

 involuted epiblast, differs from it inasmuch as it arises 

 by an independent involution of the superficial epiblast, 



