THE EYE AND EAK. 393 



3. The Ear. 



The ear of the rabbit, like that of the chick, is derived from 

 a pitting-in of the epiblast at the side of the hind-brain. By 

 closure of its mouth, the pit becomes a vesicle or sac, imbedded 

 in the mesoblast of the side of the head ; and from the walls of 

 this sac, which are of epiblastic origin, the epithelial lining of the 

 vestibule and of its various prolongations is derived ; the semi- 

 circular canals, cochlea, and other parts being formed by out- 

 growths or constrictions of the originally simple sac. 



The mesoblast immediately surrounding the sac gives rise 

 to the connective tissue wall of the auditory labyrinth, while 

 the mesoblast a little distance off gives origin to the cartilagi- 

 nous auditory capsule (cf. Fig. 159). Between the labyrinth and 

 the cartilaginous capsule a series of lymphatic spaces appear, 

 filled with fluid, in which the labyrinth hangs suspended. 

 Finally, important series of accessory organs, characteristic of 

 air-breathing Vertebrates the tympanic membrane, Eustachian 

 tube, auditory meatus, auditory ossicles, and external ear are 

 formed, and acquire definite relations with the essential organ 

 of hearing, i.e. the auditory vesicle itself. 



The auditory vesicles arise in the rabbit, towards the end of the 

 ninth day, as a pair of shallow depressions of the epiblast at the 

 sides of the hind-brain. During the tenth day each pit deepens 

 rapidly, and by the end of the day the mouth of the pit narrows 

 and closes, converting the pit into the closed auditory sac or 

 vesicle (Fig. 147, EI), which lies imbedded in the side wall of 

 the head, opposite the first branchial arch. 



The auditory vesicle is at first spherical, but soon becomes 

 triangular in outline as seen in transverse sections. The dorsal 

 angle of the triangle, which marks the place where the vesicle 

 separates from the external epiblast, grows upwards as a long 

 tubular process, the recessus vestibuli (Fig. 158, ER), which 

 follows the curvature of the brain wall, and ends blindly at its 

 dorsal extremity. 



From the outer side of the vestibule, a wide lateral diverti- 

 culum arises, from which the semicircular canals are developed 

 at a slightly later stage (Fig. 158, ED, EH). The ventral angle 

 of the vestibule is prolonged downwards and inwards as a curved 

 finger-like process, the cochlear canal (Fig. 158, EL). 



