AUDITORY ORGANS. 183 
and the outer ear, more or less completely developed, is found only in 
‘ the amniotes. 
The Inner Ear arises as a circular area of thickened ectoderm on 
either side of the head, between the seventh and ninth nerves (fig. 136). 
This soon becomes cup-shaped and then the cup closes in to form an 
auditory vesicle (fig. 183), the cavity of which is connected with the 
exterior by a slender tube, the endolymph duct, the result of incomplete 
Fic. 183.—Diagram of developing human labyrinth from 6 to 30 mm. long, after 
Streeter. @, ampulla; c, cochlear region and cochlea; au, ampullo-utricular region; d, 
endolymph duct; e, endolymph region; sc, semicircular canal; se, endolymph sac; s, sac- 
culus: #, utriculus; us, utriculo-saccular canal; v, vestibule. 
closure. As one portion of the medial wall of the vesicle develops an 
area of sensory epithelium like that of the lateral line system, this 
stage may be compared to an isolated canal organ with a single pore, 
In the amphibia and some of the ganoids, where there is a two-layered ectoderm 
from the early stages, there is never an open auditory cup. The lower, so-called 
nervous layer of the ectoderm is alone concerned in the formation of the auditory 
vesicle, while the outer layer extends as an unbroken sheet across the cup. In 
the elasmobranchs the endolymph duct opens to the exterior throughout life, the 
external pores being recognizable on the top of the head. Elsewhere they later lose 
their external openings, and the distal end of each usually expands into an enlarge- 
ment, the sacculus endolymphaticus; but in the amphibia the ducts of the two 
sides may unite dorsal to the brain, while other parts may branch and grow in a 
root-like manner, in the canal of the spinal cord, sending diverticula (frog) into the 
so-called calcareous glands, which surround the basal parts of the spinal nerves. 
The next stage in the auditory vesicle is its differentiation by a 
constriction into two chambers, an upper vestibulum or utriculus 
