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 



FIG. 183. Diagram of developing human labyrinth from 6 to 30 mm. long, after 

 Streeter. a, 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: u, 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 



