AUDITORY ORGAN 



221 



of Vertebrates is situated in the region of the head, between the 

 origins of the trigeminal and vagus nerves. After the vesicle of 

 each side has become separated off from the epiblast and connected 

 with the brain by means of the auditory nerve (which arises in 

 connection with a peripheral ectodermic ganglion and then grows 

 centripetally to 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 parts, called re- 

 spectively the utriculus .and sacculus (Fig. 177). From the former 



FiC4. 178A. ISOLATED ELEMENTS OF THE MEMBRANOUS LABYRINTH OF VARIOUS 

 VERTEBRATES. (After G. Retzius. ) 



A, from the macula acustica communis of Myxine glutinosa ; B, from the macula 

 acustica neglecta of Raia davata ; C, from the crista acustica of an ampulla 

 of Linedon (Amblystoma) mexicanu* ; D, from the crista acustica of the 

 anterior ampulla of Roma esculanta. 



hz, hair-cells with auditory hairs (h) ; fz, thread-like cells; n,n, dividing nerve. 

 On the left side of D the auditory hair has become broken up into its con- 

 stituent fibres. 



the semicircular canals become developed, while from the latter 

 the tube-like ductus endolympJiaticus and the lagena (cochlea) are 

 formed. 



The whole of this complicated apparatus constitutes the internal 

 ear or membranous labyrinth. It becomes surrounded secondarily 

 by mesoblastic tissue, with which it is at first in close contact. 

 A process of absorption then takes place in the innermost layers 

 of the mesoblast, and thus a space is developed which closely 



