VERTEBRATA. 4it 
In the frog and higher types the auditory sac becomes 
constricted into two portions called the wériculus and the 
sacculus. The utriculus gives rise to the semi-circular canals, 
and the sacculus to a coiled cochlea. The agueductus vestt- 
éuf remains closed and is known as the ductus endolym- 
phaticus. 
Fig. 297,—DEVELOPMENT OF THE VERTEBRATE Ear. 
Auditory Vesicle. B Duct. 
Utriculus, 
A 
Sacculus. 
Cc Semi-circular Canal. 
Aqueductus Semi-circular 
Vestibuli, — Canal. 
| | Ampulla, 
Utriculus. 
‘Lagena. 
Sacculus. 
A Epiblastic invagination. B, Division into superior and inferior parts. 
C, The ear as in the skate (cK Fig. 230, p. 323). 
But the most important modification is involved in the 
formation of the médd/e 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 ustachian 
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. 
