90 



DEVELOPMENT OF THE EAR. 



exterior. In elasmobranch fishes this connection is never closed, but remains 

 throughout life in the form of a small duct-like tube which passes up through the 

 cranial wall and opens on the epidermis. In other vertebrates the connection with 

 the exterior becomes closed in the chick during the third day and what remaiis 



Fig. 104. SECTIONS THROUGH REGION OP THE HIND-BRAIN OP HUMAN EMBRYOS, SHOWING THREE 



STAGES IN THE DEVELOPMENT OP THE OTIC VESICLE. 



A, auditory pits ; 13, simple auditory vesicles ; 0, auditory vesicles beginning to be fashioned into 

 parts of the membranous labyrinth. 



Fig. 105. OUTLINE OP THE RIGHT LABYRINTH OP A 3| 



WEEKS HUMAN EMBRYO, TO SHOW ITS RELATIONS TO 

 THE PARTS OP THE AUDITORY NERVE. (W. His,jun.) 



a.l, section of hind brain ; g.v, ganglion vestibuli in 

 contact with the upper part of the labyrinth ; g.c, ganglion 

 cochleae in contact with the lower part. The fibres of the 

 corresponding parts of the auditory nerve which have grown 

 from these ganglia into the hind brain, are seen to cross 

 one another ; /, facial nerve. 



of the original mouth, or canal of connection 

 with the exterior, is visible as a distinct but 

 small process from the upper and inner angle 

 of the vesicle, and is known as the recess of 

 the labyrinth (fig. 104 c, r.l). Eventually it 

 developes into a long epithelial tube, which 

 passes through the petrous bone, with an expanded end lying within the skull 

 underneath the dura mater. This tube and its expanded termination form respec- 

 tively the endolymphatic canal and saccule (fig. 10G). 



In the meantime the auditory vesicle becomes elongated and begins to be 

 irregular. Its ventral end projects as a distinct hollow process, at first straight, 

 but soon becoming curved ; this is the rudiment of the epithelial canal of the, cochlea. 



