AUDITORY ORGANS. 



427 



a number of otoliths appears in the vesicle during larval life, and, 

 although such otoliths are stated by J. Muller to be absent both in 



zr 



cv^ 



Fig. 301. 



SECTION THBOUOH THE HIND-BRAIN OF A ChICK AT THE END OF THE 

 THIRD DAY OF INCUBATION. 



IV. fourth ventricle. The section shews the very thin roof and thicker sides of 

 the ventricle. Ch. notochord ; CV. anterior cardinal vein ; CC. involuted auditory 

 vesicle {CC points to the end which will form the cochlear caual) ; RL. recessus 

 labyrinthi (remains of passage connecting the vesicle with the exterior) ; hy. hypoblast 

 lining the alimentary canal; AO., AO.A. aorta, and aortic arch. 



the full-grown Ammocoete and in the adult, they have since been 

 found by Ketel (No. 387). The formation of the two semicircular 

 canals has not been investigated. 



In all the higher Vertebrates the changes of the auditory sacks 

 are more complicated. The ventral end of the sack is produced into 

 a short process (fig. 301, CG); while at the dorsal end there is the 

 canal-like prolongation of the lumen of the sack {RL), derived from 

 the duct which primitively opened to the exterior, and which in most 

 cases persists as a blind diverticulum of the auditory sack, known as the 

 recessus labyrinthi or aqueductus vestibuli. The parts thus 

 indicated give rise to the whole of the membranous labyrinth of the 

 ear. 'i'he main body of the vesicle becomes the utriculus and semi- 

 circular canals, while the ventral process forms the sacculus hemi- 

 sphericus and cochlear canal. 



The growth of these parts has been most fully studied in Mam- 

 malia, where they reach their greatest complexity, and it will be 

 convenient to describe their development in this group, pointing out 

 how they present, during some of the stages in their growth, a form 

 permanently retained in lower types. 



The auditory vesicle in Mammalia is at first nearly spherical, and 



