VERTEBRATA. 



411 



In the frog and higher types the auditory sac becomes 

 constricted into two portions called the utficidus and the 

 sacculus. The utriculus gives rise to the semi-circular canals, 

 and the sacculus to a coiled cochlea. The aqueductus vesti- 

 huli remains closed and is known as the ductus endolym- 

 phaticus. 



Fig. 297. — Development of the Vertebrate Ear. 



Auditory Vesicle. B Duct. 



A 



/d 



Utriculus. 



Sacculus. 



Semi-circular Canal. 



Aqueductus 



Vestibuli. 



^Lagena. 



Sacculus. 

 A Epiblastic invagination. B, Division into superior and inferior parts. 

 C, The ear as in the skate {cf. Fig. 230, p. 323). 



But the most important modification is involved in the 

 formation of the middle ear. The cleft corresponding to the 

 spiracle of the skate appears to be modified into a tube, 

 closed at the surface in the frog by a membrane or tym- 

 panum, but still opening into the throat by the Eustachian 

 aperture. The hyomandibular appears to become the 

 columella which leads from the tympanum to the inner ear, 

 and transmits the vibrations of the air thereto. 



