THE INTEKNAL EAR 



51 



proceeds the plate is gradually invaginated into the substance of the head, and is 



Hind -brain Auditory ganglion 



/ / Rudiment of otic vesicle 



Xa^C^^^S-^^^X / 

 Paraxial mesoderm 



Hyomandibular cleft 



SoM 



S P M1 ' 

 First cephalic aortic arch 



SpMz 



FIG. 67. TRANSVERSE SECTION OP A BAT EMBRYO. 



Showing the relation of the paraxial mesoderm of the head to the lateral plates, the commencement of the 

 formation of the otic vesicles and hyomandibular clefts, and the relation of the primitive heart to the 

 pericardium and fore-gut. 



EC. Ectoderm. SoM. Somatic mesoderm. SpM. Splanchnic mesoderm. 



transformed into a pear-shaped vesicle, the otic vesicle, which remains for a time in 

 communication with the ex- HB 



terior by means of a short 

 tubular stalk, the recessus 

 labyrinthi, which is subse- 

 quently converted into the 

 ductus endolymphaticus. 1 



After it is separated from 

 the surface the otic vesicle 

 alters its position, until its 

 ventral end lies in close re- 

 lation to the dorsal wall of 

 the pharynx, and, at the same 

 time, it undergoes alteration 

 of shape. The ventral part 

 of the vesicle grows towards 

 the median plane, along the 

 ventral wall of the hind-brain. 

 It forms the cavity and the 

 lining epithelium of the coch- 

 lea; but it remains in con- 

 nexion with the dorsal part 

 by means of a narrow tube, 

 the canalis reuniens, and as it 

 grows in length it becomes 

 converted into a spiral tube. 



The portion of the dorsal 

 section of the primitiv e vesicle, 

 which lies to the lateral side 

 of the recessus labyrinthi, first 



HM 



FIG. 68. TRANSVERSE SECTION THROUGH THE HEAD 

 OP AN EMBRYO. 



Showing the rudiments of the three parts of the ear and their 

 relation to the hyomandibular cleft. 



BV. Blood-vessels. 



C. Cochlea. 



EM. Ext. acoustic meatus. 



ET. Auditory tube. 



HB. Hind-brain. 



HM. Hyomandibular cleft. 



N. Notochord. 

 0V. Otic vesicle. 

 P. Pharynx. 



Kecessus labyrinthi. 



Semicircular canal. 



Tympanum. 



RL. 



SC. 

 T. 



expands and 



1 See note 3, p. 79. 



then becomes compressed and 



