FIG. 404. 



NORMAL HISTOLOGY. 



of the ectodermic otic vesicle, since from this is produced the im- 



portant membranous tube, the enveloping fibrous and osseous 

 structures being comparatively late contri- 

 butions from the mesoderm. 



The internal ear first appears as a thick- 

 ening and soon after depression of the 

 ectoderm within a small area on either 

 side of the cephalic end of the neural tube at 

 a level corresponding to about the middle 

 of the future medulla. This auditory pit 

 is widely open for a considerable time and 

 distinguished by the great thickness of its 

 depressed wall, which contrasts strongly 

 with the adjacent ectoderm. After a time 

 the lips of the pit approximate until by 

 their final union the cup-like depression 

 is converted into a closed sac, the otic 

 vesicle. 



The otic vesicle, after severing all con- 

 nection with the ectoderm, gradually re- 

 cedes from the surface in consequence of 

 the growth of the intervening mesodermic 



layer ; it next loses its spheroidal form and becomes pear-shaped, 



with the smaller end directed 



dorsally. This diverticulum FIG. 406. 



is the first appearance of the 



recessus vestibuli, a divi- 



sion of the embryonal laby- 



FIG. 405. 



Section through developing ear 

 of nine-and-a-half-day rabbit em- 

 bryo : e, ectoderm thickened and 

 invaginated to form auditory pit 

 at a; m, surrounding still undif- 

 ferentiated mesoderm ; n, lining 

 of neural tube ; z>, blood-vessel. 



Sagittal section through developing ear 

 of ten-day rabbit embryo : o, otic vesicle 

 becoming pear-shaped, due to formation 

 of recessus vestibuli (r) ; m, surrounding 

 mesoderm. 



Section through developing ear of twelve-day 

 rabbit embryo : v, primitive vestibule, from 

 which extend (r) recessus labyrinthi and (s) 

 semicircular canal above and (c\ cochlear canal 

 below ; n, neural tube with thickened ventral 

 lining ; m, mesoderm. 



rinth disproportionately conspicuous compared with its permanent 

 representative, the ductus endolymphaticus. 



