296 THE DEVELOPMENT OF THE CHICK 



facialis ganglion which lies in front of and below the center 

 of the auditory pit. During the closure of the latter, the acustic 

 ganglion becomes fused with part of the wall of the otocyst in 

 such a way that it becomes impossible to tell in ordinary sec- 

 tions where the epithelial cells leave off and the ganglionic cells 

 begin. This fused area may be called the auditory neuro-epi- 

 thelium. At the 36 somite stage the neuro-epithelium is confined 

 to the lower (ventral) fourth of the otocyst, covering the entire 

 tip, the anterior face, and a small portion of the median face 

 (cf. Fig 98). The neuro-epithelium is the source of all the sen- 

 sory areas, which arise from it by growth and subdivision. The 

 branching of the auditory nerve follows the subdivision of the 

 neuro-epithelium. 



The exact manner in which the changes take place has not 

 been made a subject of special investigation in the chick, so far 

 as the author knows. However, it can be said in general that 

 there is first a partial division of the neuro-epithelium into a 

 pars superior and a pars inferior, and that the former divides 

 into the cristse acusticae (sensory areas of the three ampullae) 

 and the macula utriculi, while the latter furnishes the macula 

 sacculi, papilla basilaris and papilla lagense. 



The sensory cells differentiate from the epithelium of the 

 labyrinth, and the nerve fibers from the bipolar neuroblasts of 

 the acustic ganglion, the peripheral process growing into the 

 epithelium and branching between the sensory cells, wdiile the 

 central process grows into the brain. 



(e) Bony Labyrinth, Perilymph, etc. The loose mesenchyme 

 that entirely surrounds the otocyst, differentiates in the course 

 of development into the membrana propria and perilymphatic 

 tissue of the membranous labyrinth, the perilymph and the bony 

 labyrinth in the folio w^ing manner; on the sixth day a single layer 

 of mesenchyme cells in contact with the cells of the otocyst are 

 arranged with their long axes parallel to the wall, and show 

 already in places a slight fibrous differentiation. These gradually 

 form the membrana propria, which appears on the eighth day 

 as an extremely thin adherent layer with protruding nuclei at 

 intervals. The mesenchyme external to this delicate layer is 

 already differentiated on the sixth day into a perilymphatic 

 and a procartilaginous zone; in the former the mesenchyme is 

 of loose consistency, and in the latter zone it has become dense 



