AUDITORY ORGAN. 195 



original form of both auditory and lateral line organs is a vesicle 

 derived from the epiblast. from which it later becomes separated 

 off; it is lined by elongated cells of sensory epithelium provided 

 with hair-like processes (auditory hairs), and by supporting cells. 

 Moreover, both structures are supplied by cranial nerves (VIII, 

 JT) which correspond to dorsal roots. 



Like the other higher sense-organs, the paired auditory organ 

 of Vertebrates is situated in the region of the head, and it always lies 

 between the origins of the trigeminal and vagus nerves. The first 

 truces of it in the embryo are seen to the right and left of the hind 

 brain (Fig. 160, LB], and after the vesicle of each side has be- 

 c< >me separated off from the epiblast and connected with the auditory 

 nerve which grows out towards it from the brain, it sinks deeper 

 and deeper into the mesoblastic tissue of the skull : it then loses its 

 original pyriform or rounded shape, and becomes divided into two 



FIG. 160. HEAD AND ANTERIOR PORTION OF BODY OF A CHICK. (In part after 



Moldenhauer. ) 



EG, olfactory pit ; A, eye ; I to IV, first to fourth visceral arches ; +, point at which 

 the external auditory passage begins to be formed ; LB, primitive auditory 

 vesicle seen through the wall of the head. 



parts, called respectively the utriculus and sacculus (Fig. 161, Ut, 

 S), From the former the semicircular canals become differen- 

 tiated, while from the latter the tube-like recessus vestibuli 



(aquyeductus vestibuli s. ductus endolymphaticus) and the cochlea 

 are formed (Fig. 161, S.B., F.B., H.B.,D.e, C}. 



This whole, very complicated, apparatus constitutes the mem- 

 branous auditory organ or membranous labyrinth. It 

 becomes surrounded secondarily by mesoblastic tissue, which is at 

 first in close contact with it ; later, however, a process of absorption 

 takes place in the innermost layers of the mesoblast, thus giving 

 rise to a space, which closely repeats the form of the membranous 

 labyrinth, as does also the mesoblast which encloses this space, and 

 which later becomes chondrified, and often also ossified. We thus 



o -2 



