AUDITORY ORGANS. 



517 



branchii) remaining open. The walls of this vesicle are always 

 much complicated and otoliths of various forms are presentjnjts 

 cavity. To this vesicle accessory structures, derived from the 

 walls of the hyomandibular cleft, are added in the majority of 

 terrestrial Vertebrata. 



The development of the true auditory vesicle will be considered 

 separately from that of the accessory structures derived from the 

 hyomandibular cleft. 



In all Vertebrata the development of the auditory vesicle 

 commences 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 previous chapter is primi- 

 tively a branch of the seventh nerve, 

 closely applies itself. 



In those Vertebrata (viz. Teleostei, Le- 

 pidosteus and Amphibia) in which the epi- 

 blast 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 (fig. 300) by the epidermic 

 stratum of the skin. Since the opening of 

 the pit is retained through life in Elasmo- 

 branchii 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. 



Th 



FIG. 299. SECTION THROUGH 

 THE HEAD OF AN El.ASMOBRANCH 

 EMBRYO, AT THE LEVEL OF 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.aa. 

 aortic trunk of mandibular arch ; 

 //. head cavity of mandibular 

 arch ; Ivc . alimentary pouch which 

 will form the first visceral cleft; 

 Th. rudiment of thyroid body. 



The mouth of the auditory vesi- 

 cle gradually narrows, and in most 



forms soon becomes closed, though in Elasmobranchii 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 (Elasmo- 

 branchii), or ending blindly close beneath the skin. 



In all Vertebrata the auditory vesicle undergoes further 



