464 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



The first sign of the development of the ear is the 

 formation, on either side of the hinder part of the head, 

 of a pit or depression (Fig. 196 ; au v), which gradually 

 deepens, and with which an outgrowth from the audi- 

 tory nerve (au n) comes into contact. The pit becomes 

 converted into an elongated sac, narrower above and 

 below. The upper end grows out and gives rise to 

 the recess of the membranous labyrinth, the lower 



77 



o' 



Fig. 195. Larva of Ascidia mentula ; the Anterior portion of the tail is 



alone represented. 



Hi, Anterior swelling of neural tube; N, anterior swelling of spinal portion of 

 neural tube ; n, hinder part of neural tube ;' cli, notochord; K, Abranchial 

 region of alimentary tract ; d, its cesophageal and gastric region ; o, eye ; a, 

 otolith ; o', mouth ; s, papilla of attachment. (After KuppfEer.) 



end forms the cochlear canal, and the narrow duct 

 (canalis reuniens) by means of which the cochlear 

 canal communicates with the sacculus ; the median 

 portion of the auditory vesicles gives rise to the semi- 

 circular canals, and to the utriculus. 



Fig. 197 (page 466) will make clear the relations 

 of the just named and of other parts in the complex 

 auditory organ of man. 



External to the proper sensory portion in which 

 the branches of the auditory nerves terminate in the 

 special sensory cells, there is a median division of the 

 ear, in which are developed one or more bones that 

 convey the sonorous vibrations, and these are acted 



