108 



ZOOLOGY 



SECT. 



ectoderm in the region of the hind-brain, a shallow depression being 

 formed which deepens and becomes flask-shaped, and finally, as a 

 rule, loses its connection with the external ectoderm, becoming, 

 a closed sac surrounded by mesoderm. At first simple, it soon 

 becomes .divided by a constriction into dorsal and ventral com- 

 partments, The dorsal compartment is differentiated into an 

 irregular chamber, the utriculus (Fig. 744, u.), and, usually, three 

 tubes, the semicircular canals. Of these two, the anterior (ca.) 





><m& posterior (ap.) canals, are vertical in position and have their 

 adjacent limbs united so that the two canals have only three 

 openings between them, into the utriculus: the third or external 

 canal (ae.) is horizontal, and opens into the utriculus at either 

 end. Each canal is 'dilated at one of its ends into an ampulla 

 (ae., ac., ap.\ placed anteriorly in the anterior and external canals, 

 posteriorly in the posterior canal. 



The ventral compartment of the auditory sac is called the 

 sacculus (,<?.) : it gives off posteriorly a blind pouch, the cochlea (7.), 



