THE ORGANS OF THE OUTER GERM-LAYER. 505 



and are stretched out smoothly between the free margin of the lamina 

 spiralis and the inner wall of the cartilaginous capsule. In this process 

 the tympanic wall (C) comes to lie in the same plane as the lamina 

 spiralis, the vestibular wall (Iv) forms with the tympanic an acute 

 angle, and the third wall (x) is everywhere in close contact with the 

 perichondrium of the cartilaginous capsule. 



The epithelial lining of the membranous duct us cochlearis assumes 

 very different conditions in the three corresponding regions of its 

 wall. Whereas the epithelial cells of the vestibular and the outer 

 walls become in part cubical, in part quite flat, those of the tympanic 

 wall become elongated, and are in connection with the terminal fila- 

 ments of the cochlear nerve ; they produce the complicated organ of 

 CORTI (C), which, like the auditory ridges and auditory patches of 

 the ampullre, the sacculus and utriculus, contains the terminal ends 

 of the auditory nerve. 



The construction of the intricate cochlea approaches completion 

 with the beginning of the process of ossification. The latter is accom- 

 plished by two methods. First, the cartilaginous capsule ossifies in 

 the endochoiidral manner, as does the whole cartilaginous os petrosum, 

 of which it constitutes a small part. The osseous tissue thus formed 

 is for a long time spongy and provided with large medullary spaces. 

 Secondly, the previously mentioned fibrous connective-tissue layers- 

 the partitions between the cochlear canals, the connective-tissue 

 axis or the niodiolus and the lamina spiralis undergo direct ossifi- 

 cation. At the same time compact bone-lamell?e are laid down from 

 within on the spongy bone-tissue formed from the cartilaginous capsule; 

 these lamelke are formed, as BOETTCHER has shown, by the original 

 perichondrium, which becomes the periosteum. Consequently the 

 bony cochlear capsule, since it is produced by periosteal secretion, 

 may be easily detached from the loose osseous tissue of endochoiidral 

 origin during early post-natal years. 



(c) Development of the Accessory Apparatus of the Organ of Hearing. 



(Middle and External Ear.} 



With the membranous and bony labyrinth, which are together 

 called the inner ear, there is associated a subsidiary apparatus, in the 

 same way that the eye-muscles, the lids, and the lachrymal glands 

 and ducts are added to the eyeball. It is made up of structures 

 which are wanting in the lower Vertebrates (Fishes), but, beginning 

 to be developed in the Amphibia, become more and more complete in 



