THE ORGANS OF THE OUTER GERM-LAYER. 



507 



tion of the pharynx, this view is opposed not only to comparative- 

 anatomical considerations, but also to statements of KOLLIKER, 

 MOLDENHAUER, and HOFFMANN, which relate to the development in 

 Keptiles, Birds, and Mammals. 



In the classes of Vertebrates just mentioned the first visceral 



Fig, 284. -Diagrammatic representation of the whole organ of hearing in Man, from WIEDERSHEIM. 



Outer ear: M, M, auricle; Mae, meatus auditorius externus ; O, its wall; Mt, membrana 

 tympani. Middle ear: Ct, Ct, cavum tympani ; O 1 , its wall; SAp, sound-conducting 

 apparatus, which is drawn as a simple rod-like body in place of the auditory ossicles ; the 

 place t corresponds to the stapedial plate, which closes the fenestra ovalis ; Tb, tuba 

 Eustachii ; Tb 1 , its opening into the pharynx ; 0", its wall. Inner ear : the bony labyrinth 

 (KL, KL 1 ) for the most part cut away ; S, sacculus ; a, b, the two vertical membranous and 

 osseous semicircular canals ; S.e, D.e, saccus and ductus endolymphaticus, of which the 

 latter is divided at 2 into two arms ; Cp, cavum perilymphaticum ; Or, canalis reunions ; 

 Con, membranous cochlea, which produces at + the vestibular coecum ; Con 1 , bony cochlea ; 

 Sv and St, scala vestibuli and scala tympani, which at * communicate with each other at the 

 cupula terminalis (Ct) ; D.p, ductus perilymphaticus, which arises from the scala tympani 

 at d and opens out at D.p 1 . The horizontal semicircular canal is not specially designated, 

 but is easily recognisable. 



cleft is closed in its upper part also, contrary to the condition in 

 Selachians.* 



The closure becomes more firm and complete owing to the in- 

 growth of a connective-tissue layer between the inner and outer 

 epithelial plates. Remnants of the first visceral cleft are preserved 



* See the statements discussed in a previous chapter (p. 287), concerning 

 the mooted question whether the visceral clefts remain closed by means of 

 an epithelial membrane or are temporarily open. 



