598 TEXT-BOOK OF EMBRYOLOGY. 



duct between the latter and the scala tympani (Fig. 514). The epithelial cells 

 of the cochlear duct in this region become high columnar and arranged in two 

 ridges which extend throughout the entire length of the duct. The cells of the 

 ridge nearer the axis of the cochlea give rise to the membrana tectoria. Whether 

 this is accomplished by cuticular secretion of the cells or by the fusion of long 

 hair-like processes that grow from their free borders is not known. The cells of 

 the outer ridge become differentiated into four groups. Those of the outer 

 group (next the cells that give rise to the membrana tectoria) develop into the 

 inner hair cells; those of the next group form the pillar cells; those of the third 

 group differentiate into the outer hair cells; and those of the fourth (outer) 

 group give rise to Hensen's cells. The hair cells, as the name indicates, develop 

 delicate hair-like processes on their free borders, and, since the peripheral 

 processes of the spiral (cochlear) ganglion cells end around them, are con- 

 sidered as the sensory cells of the cochlea, or auditory receptors (see p. 506). 



THE ACOUSTIC NERVE. The acoustic ganglionic mass is at first closely 

 associated with the geniculate ganglion (ganglion of the facial (VII) nerve), the 

 two together often being spoken of as the acustico-facialis ganglion (see also 

 p. 505). This lies in close contact with the anterior wall of the auditory vesicle 

 when the latter is first constricted from the ectoderm. The origin of the gang- 

 lion has not been traced in Mammals, but in cow embryos the geniculate has 

 been seen to be connected with the ectoderm at the dorsal end of the first 

 branchial groove (Froriep). The acoustic ganglion probably belongs to the 

 lateral line system (Kupffer) (see also p. 467). 



Although the geniculate and acoustic ganglia are at first closely associated, 

 each pursues an independent course of development. The description here 

 will be confined to the acoustic. As already mentioned, this lies in close apposi- 

 tion to the side of the neural tube and the auditory vesicle and just anterior to 

 the latter (Fig. 512, a). At a very early stage (embryos of 6-7 mm.), the mass 

 shows a differentiation into two parts a dorsal one, the future vestibular 

 ganglion, and a ventral one, the future cochlear (spiral) ganglion (Fig. 512, b, c). 

 The ganglion cells become bipolar (see p. 506), and, as is peculiar to the cells of 

 the acoustic ganglia, remain in this condition. One process of each cell grows 

 centrally to form a root fiber of the acoustic nerve, which terminates in contact 

 with dendrites of neurones in certain nuclei in the central nervous system. The 

 fibers from the cells of the vestibular ganglion form the vestibular root, those 

 from the cells of the cochlear ganglion form the cochlear root. The other proc- 

 ess grows peripherally and penetrates the wall of the auditory vesicle to enter 

 into relation with certain cells that differentiate from the epithelial lining of the 

 vesicle. 



The peripheral processes of the vestibular ganglion cells come into relation 

 with specialized cells (hair cells) in the ampullae of the semicircular canals 



