398 



of the ectodermic otic 

 portant membranous 



FIG. 404. 



NORMAL HISTOLOGY. 



vesicle, since from this is produced the im- 

 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- 46. 



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 o ; tn, surrounding still undif- 

 ferentiated mesoderm ; n, lining 

 of neural tube ; v, 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 labyrinth! and (s) 

 semicircular canal above and (c) cochlear canal 

 below ; , neural tube with thickened ventral 

 lining ; m, mesoderm. 



rinth disproportionately conspicuous compared with its permanent 

 representative, the ductus endolymphaticus. 



