EAR 



145 



DEVELOPMENT OF THE EAK. 1 



The essential part of the ear viz. the epithelial lining of the labyrinth is 

 developed in much the same way as the crystalline lens, as an invagination of the 

 external ectoderm, which at first appears as a pit of thickened epithelium (auditory 

 pit, fig. 177), but is gradually converted by a growing together of the margins of 

 the pit into a hollow island of ectoderm, the auditory or otic vesicle (fig. 180). This 

 process occurs somewhat after the formation of the eye has begun, and at quite 

 a different part of the head viz. on either side of the hind-brain just over the 

 upper end of the first post- oral visceral cleft. The vesicle comes at first into close 

 contact with the hind-brain, except where the ganglionic rudiment of the auditory 

 nerve projects between them, but it subsequently becomes entirely surrounded 

 by mesoderm, which separates it from both the neural and external ectoderm. 



c.c. 



FIG. 186. STAGES IN THE DEVELOPMENT OP THE MEMBRANOUS LABYRINTH. (W. His, Jr.) 



A. Left labyrinth of a human embryo of about four weeks, viewed from the outer side, u, vestibular 

 part ; c, cochlear part ; r.L, recessus labyrinthi (aqueductus vestibuli). 



B. Left labyrinth with parts of the facial and auditory nerves of a human embryo of about four and 

 a-half weeks, b, surface of the hind-brain ; u, utricular ; s, saccular part of labyrinth; a.u.c., p.s.c.,e.s.c., 

 rudimentary folds representing the two vertical and the horizontal semicircular canals ; r.L, upper part 

 of recessus labyrinthi becoming enlarged into the endolymphic saccule ; c.c., rudiment of cochlea ; n.v., 

 vestibular branch of auditory nerve ; g.v., vestibular ganglion (ganglion of Scarpa) ; g.c., cochlear 

 ganglion ; n.f., facial nerve, with geniculate ganglion, g.g. 



C. Left labyrinth of a human embryo of about five weeks, viewed from without and below. Letter- 

 ing as before. The horizontal canal is still only a fold. The ampullae are beginning to be visible on 

 the two vertical canals. 



The otic vesicle is at first flask- shaped, with the somewhat elongated mouth of 

 the flask directed externally towards the original point of connexion with the 

 exterior. In elasmobranch fishes this passage 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 opening to 

 the exterior becomes closed, and what remains of the original mouth, or canal of 

 connexion with the exterior, is visible as a distinct but small process from the 



1 For literature see Krause, Hertwig II. Th. i. and ii. p. 133 seq. 

 in footnotes. 

 VOL. I. 



Reference to more recent papers 



