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 lo^e^ its connection with the external ectoderm, becoming 

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

 become* 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.) 



. 

 "-: V-V '*. 



St 



Fi,.. 743.-Section of the pineal eye of Hatteria. g, blood-vessel ; h, cavity of eye, filled with 

 fluid ; A-, connective tissue capsule ; L lens ; M. molecular layer of retina ; r, layer of rods and 

 cones ; st, nerve ; x, cells in nerve. (From Wiedersheim's Vvrtelrata, after Baldwin Spencer.) 



><mdi 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 (ac.} is horizontal, and opens into the utriculus at either 

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

 (ac., 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 

 wwulus (s.) : it gives off posteriorly a blind pouch, the cochlea (L), 



