AUDITORY ORGANS OF THE VERTEBRATA. 



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In all Vertebrata the development of the auditory vesicle com- 

 mences with the formation of a thickened patch of epiblast, at the 



side of the hind-brain, on the level of 

 the second visceral cleft. This patch 

 soon becomes invaginated in the form 

 of a pit (fig. 299 aup), to the inner side 

 of which the ganglion of the auditory 

 nerve (aun), which as shewn in a pre- 

 vious chapter is primitively a branch of 

 the seventh nerve, closely applies itself. 



In those Vertebrata (viz. Teleostei, Le- 

 pidosteus and Amphibia) in which the 

 epiblast is early divided into a nervous 

 and epidermic stratiim, the auditory pit 

 arises as an invagination of the nervous 

 *tratum only, and the mouth of the auditory 

 pit is always closed (tig. 300) by the epi- 

 dermic stratum of the skin. Since the 

 opening of the pit is retained through life 

 in Elasmobranchii the closed form of pit 

 in the above forms is clearly secondary. 



In Teleostei the auditory pit arises as 

 a solid invagination of the epiblast. 



The mouth of the auditory vesicle 

 aortic trunk of mandibular arch; gradually narrows, and in most forms 



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-J.etff 



FIG. 299. SECTION THROUGH 

 THE HEAD OF AN ELASMOBRANCH 

 EMBRYO, AT THE LEVEL OF THE 

 AUDITORY INVOLUTION. 



(inp. auditory pit; aim. gan- 

 glion of auditory nerve; io.r. roof 

 of fourth ventricle; a.c.v. anterior 

 cardinal vein; aa. aorta; I.ai. 



pp. head cavity of mandibular 

 arch; Inc.. alimentary pouch which 

 will form the first visceral cleft ; 

 Th. rudiment of thyroid body. 



soon becomes closed, though in Elasmo- 

 branchii it remains permanently open. 

 In any case the vesicle is gradually 

 removed from the suiface, remaining 



connected with it by an elongated duct, either opening on the dorsal 

 aspect of the head (Elasmobranchii), or ending blindly close beneath 

 the skin. 



In all Vertebrata the auditory vesicle undergoes further changes 



of a complicated kind. In the 

 Cyclostomata these changes 

 are less complicated than in 

 other forms, though whether 

 this is due to degeneration, or 

 to the retention of a primitive 

 state of the auditory organ, is 

 not known. In the Lamprey 

 the auditory vesicle is formed 

 in the usual way by an in- 

 vagination of the epiblast, 

 which soon becomes vesicular, 

 and for a considerable period 



Auditory Vesicle ; au.n. auditory retains a simple character. As 

 nerve; ch. notochord ; hy. hypoblast. pointed out by Max Scllllltze, 



FIG. 300. SECTION THROUGH THE HEAD OF 

 A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY 

 AFTER IMPREGNATION. 



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