5 i8 



AUDITORY ORGANS OF THE VERTEBRATA. 



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 



FIG. 300. SECTION THROUGH THE HEAD OK A LEPIUOSTEUS EMBRYO ON 



THE SIXTH DAY AFTER IMPREGNATION. 

 au.v. auditory vesicle ; au.n. auditory nerve ; ch. notochord ; hy. hypoblast. 



state of the auditory organ, is not known. In the Lamprey the 

 auditory vesicle is formed in the usual way by an invagination 



cv 



cc 



AOA 



FIG. 301. SECTION THROUGH THE HIND-BRAIN OK 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 ; C V. anterior cardinal vein; CC. involuted auditory 

 vesicle (CC points to the end which will form the cochlear canal) ; 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. 



