426 



AUDITORY ORGANS OF THE VERTEBRATA. 



In all Vertebrata the development of tlie 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 tliose Vertebrata (viz. Teleostei, Le- 

 pidosteus and Amphibia) in which the 

 epiblast is early divided into a nervous 

 and epidermic stratum, the auditory pit 

 arises as an invagination of the nervous 

 stratum only, and the mouth of the auditory 

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

 dermic stratum of the skin. Since the 

 openii)g 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 

 gradually narrows, and in most forms 

 soon becomes closed, though in Elasmo- 

 branchii it remains permanently open. 

 In any case the vesicle is gradually 

 removed from the surface, 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- 

 ch bv vagination of the epiblast, 

 Fig. 300. Section thkough the head of -yvhich SOOn becomes vesicular, 



A LePIDOSTEUS embryo on the sixth day 1 i? "IT.! -J 



AFTER impregnation. ^ud for d. cousidcrable period 



au.v. auditory vesicle; a«.n. auditory retams a Simple character. As 



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



Th 



Fig. 299. Section through 



THE head op an ElASMOBRANCH 

 embryo, at THE LEVEL OP THE 

 AUDITORY INVOLUTION. 



aup. auditory pit; aun. gan- 

 glion of auditory nerve; iv.v. roof 

 of fourth ventricle ; a.c.v. anterior 

 cardinal vein ; aa. aorta ; I.ai. 

 aortic trunk of mandibular arch ; 

 pp. head cavity of mandibular 

 arch; /r-c. alimentary pouch which 

 will form the first visceral cleft ; 

 Th. rudiment of thyroid body. 



