296 THE DEVELOPMENT OF THE CHICK 
facialis ganglion which hes 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 criste acustice (sensory areas of the three ampullz) 
and the macula utriculi, while the latter furnishes the macula 
sacculi, papilla basilaris and papilla lagene. 
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, while 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 perilymphatie 
tissue of the membranous labyrinth, the perilymph and the bony 
labyrinth in the following 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 
