CHAP. iv.J HEARING. 177 



and the subsidiary dioptric mechanism is built up around the optic 

 vesicle ; and in a somewhat similar way the auditory epithelium is 

 developed into an otic vesicle, and the subsidiary acoustic apparatus 

 is built up around the otic vesicle. The otic vesicle, like the optic 

 vesicle, is lined by an epithelium of epiblastic origin, but is not like 

 that vesicle budded off from the medullary canal. It takes origin 

 in an involution of the skin covering the head ; for a time the 

 epithelium of the vesicle is continuous with the epidermis of the 

 ^.kin, and wholly unconnected with the developing brain ; later on 

 the epithelial involution separates from the skin, becomes a closed 

 independent vesicle, and makes connections with the brain through 

 the auditory nerve growing out to meet it. The otic vesicle 

 therefore is not like the optic vesicle a part of the brain, and we 

 find accordingly the structure of the auditory epithelium much 

 more simple than that of the retina ; it corresponds only to a part 

 of the retina, to the more external layers of the retina, not to all of 

 them. 



We have seen that the optic fibres are connected with a part 

 only of the optic vesicle, with the anterior wall only of the retinal 

 cup and not with the whole of this ; the part of the anterior wall 

 which forms the pars ciliaris retinae and the whole of the posterior 

 wall make no connections with the optic fibres and remain in the 

 form of a relatively simple epithelium. The connection of the 

 auditory nerve with the walls of the otic vesicle is still more 

 partial ; the nerve fibres become connected with the epithelium in 

 a few limited areas. It is only in these areas that the epithelium 

 lining the otic vesicle becomes differentiated into the special 

 auditory epithelium; elsewhere it possesses relatively simple 

 characters. 



The cavity of the optic vesicle is, as we have seen, soon 

 obliterated by the coming together of the anterior and posterior 

 walls. The cavity of the otic vesicle is permanent, growing with 

 the growth of the organ and becoming filled with a peculiar fluid 

 secreted by the walls, called endolympli. The vesicle as it grows 

 soon loses its early simple, more or less spherical form and assumes 

 a most complicated shape, becoming divided into the parts known 

 as the utricle with the semicircular canals, the saccule, and the 

 canalis cochlearis ; of these we shall speak in detail later on. 



809. While the vesicle is assuming this complicated shape, 

 the mesoblastic tissue investing it undergoes a differentiation. 

 The tissue immediately in contact with the epithelium becomes 

 connective tissue serving as a dermis to support the epithelium, so 

 that the vesicle becomes a (complicated) sac with membranous 

 walls lined with epithelium specially modified into auditory 

 epithelium at particular places, at which places and at which 

 places alone, the auditory nerve makes connections with the 

 walls. 



The outer portion of the mesoblastic tissue is converted into 



12 



