3 82 



THE PERIPHERAL NERVOUS SYSTEM 



The Inner Ear. — The epithelium of the internal ear is derived from the ecto- 

 derm. Its first anlage appears in embryos of 2 mm. as thickened ectodermal 

 plates, the auditory placodes (Fig. 363 A). These are developed dorsal to the 

 second branchial grooves at the sides of the hind-brain opposite the fifth neuro- 

 meres (Fig. 364) . The placodes are invaginated to form hollow vesicles which close 



Auditory ganglion 



Auditory placode 



Otic vesicle 



Hind-brain 



Otic vesicle 



Fig. 363. — Two stages in the early development of the internal ear. A, section through the head 

 of a 2 mm. embryo showing the auditory placode and otic vesicles; B, section through the hind-brain 

 and otic vesicles of an early human embryo (Keibel and Elze). 



Ectoderm 



Wallofhind brmn 



/Veur.5 



Fig. 364. — Four sections through the right otic vesicle of an early human embryo, r. e., endo- 

 lymphatic recess, the anlage of the endolymph duct and sac; 0. v., otic vesicle; Ncur. 4, Neur. j, neuro- 

 meres four and five of the myelencephalon (Keibel and Elze) . About 30 diameter. 



in embryos of 2.5 to 3 mm., but remain attached to the ectoderm for some time 

 (Fig. 363 B). 



The auditory vesicle or otocyst when closed and detached is nearly spherical, 

 but at the point where it was attached to the ectoderm a recess is formed. The 

 point of origin of this recess is shifted later from a dorsal to a mesial position and 

 it constitutes the ductus endolymphaticus (Figs. 365 and 366 a). The endolymph 



