714 MANUAL OF PHYSIOLOGY. 



cochlea becomes separated from the latter by a narrow constric- 

 tion, which forms the canalis reuniens, the part of the vesicle 

 beyond this developing into the saccule. 



The utricle arises from that part of the vesicle which is in con- 

 nection with the semicircular canals. It is at first in direct con- 

 nection with the saccule, but after a time it is only found to com- 

 municate by means of a narrow canal with a similar one from 

 the saccule ; these two canals are connected with a third, which 

 lies in the aqueductus vestibuli. 



The canal of the cochlea is at first a straight tube, but as it 

 develops it becomes coiled upon itself. 



The walls of the primitive otic vesicle, formed from the epiblast, 

 become developed into the epithelium lining the internal ear. 

 The mesoblast immediately surrounding the vesicle forms a sup- 

 porting capsule of fibrous tissue, which completes the membranous 

 parts of the internal ear. 



Part of the mesoblast around the otic vesicle becomes liquefied, 

 and gives origin to the canals and spaces in which the membran- 

 ous labyrinth lies ; the neighboring mesoblast is changed into 

 cartilage, which ossifies and forms the bony parts of the ear. 



The auditory nerve is developed from the hind-brain, and 

 grows through the mesoblast toward the otic vesicle. It is recog- 

 nizable from its having some ganglion cells in its growing ex- 

 tremity from a very early period of its development. 



The Eustachian tube and the tympanum, or cavity of the 

 middle ear, arc formed in connection with the inner part of the 

 first visceral cleft, and the ossicles are developed from the corre- 

 sponding visceral arch, namely, the hyo-mandibular. 



The membrana tympani is formed at the surface of the embryo ; 

 the adjacent parts grow outward and give rise to the external 

 auditory meatus. 



DEVELOPMENT OF THE SKULL AND FACE. 



The bones of the roof of the skull and of the face are chiefly 

 derived from membrane, those of the base of the skull being laid 

 down in cartilage. 



At the cephalic extremity of the notochord is a mass of uncleft 



