THE EAR OF MAN. 



207 



former structures seen from inside the vesicle are 

 merely depressed grooves in the wall of the vesicle, and 

 the latter a sunken but broadly open pit. These grooves 

 grow deeper, their edges grow together, first, along the 

 middle part of their course, thereby producing canals. 



The lips of the pit fuse by reaching across the middle 

 of the opening, and thus produce the beginning of the 

 posterior canal, which, by its continued growth upward, 

 soon reaches and fuses with the anterior canal. The 

 terminal pores of these two canals, at what proves from 

 later events to be their amal ends, fuse and open into 

 the vesicle by a single pore. 



Fig. 13. — The left internal ear from a human 

 embryo, 13 mm. long, about the fifth week of 

 development. 



The introductory stages in the development of the 

 human ear are but a repetition, as regards essentials, of 

 the process as I have described it in the shark. 



When fully cut off from the exterior, the auditory 

 vesicle forms a compressed sac having an irregular 



