1184 



EMBRYOLOGY. 



oped from the " neural crest " in the manner above described (page 1180), pierces 

 the auditory capsule in two main divisions one for the vestibule, the other for the 

 cochlea. The middle ear and Eustachiari tube are the remains of the inner part 

 of the first branchial cleft (hyomandibular), and are closed externally by the mem- 

 brana tympani, which originally consists of a layer of epiblast externally, and a 

 layer of hypoblast internally ; between these two layers the mesoblast extends to 

 form the substantia propria of the membrane. With regard to the exact mode 



Recessus labyrinthi 

 (aqitseductus vestibuli). 



Hind-brain. 



f-J--O/ic rexicle. 



Fiu. 736. Section through hind-brain 

 and otic vesicle of an embryo more ad- 

 vanced than that of Fig. 735. (After His.) 



Auditory vesicle. 



FIG. 737. Left auditory vesicle of a human embryo of four 

 weeks, seen from the outer surface. (W. His, Jr.) 



of development of the ossicles of the middle ear there is considerable difference 

 of opinion. The most probable view is that the incus and malleus are developed 

 from the proximal end of the mandibular (Meckel's) cartilage (Fig. 719) : that 

 the base of the stapes is formed by the ossification of the cartilage which fills in 

 the foramen ovale and its arch from the ossified proximal end of the hyoidean arch. 

 The external auditory meatus is formed from the outer part of the hyo- 



Recessuti labyrinthi. 



Superior semicircular canal. 



Posterior semi- 

 circular canal. 

 External semi- 

 circular canal. 



Rudiment of cochlea. 

 FIG. 738. Left auditory vesicle of a human embryo of five weeks, seen from the outer surface. (W. His, Jr.) 



mandibular cleft, while the pinna is developed by the gradual differentiation of a 

 series of processes which appear around the outer margin of the cleft (Fig. 741). 

 Development of the Nose. The olfactory fossae, like the primary auditory vesi- 

 cles, are formed in the first instance by a thickening and involution of the epiblast, 

 ,vhich takes place about the fourth week, at a point below and in front of the ocular 

 vesicle (Fig. 723). The borders of the involuted portion very soon become promi- 

 nent, in consequence of the development of the mesial and lateral nasal processes 

 already referred to (page 1169), and which are formed on either side of the rudi- 

 mentary fossa (Figs. 714, 715). As these processes increase, the fossa deepens 



