DEVELOPMENT OF THE SENSE ORGANS 



38s 



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 a thickened ectodermal 



Ectoderm 



-.r-iVe«r. 5 



Neur. s 



Fig. 383. — Four sections through the right otic vesicle of a 4 mm. human embryo (after Keibel and 

 Elze). X about 30. r.e., Endolymphatic recess, the anlage of the endolymph duct and sac; o.v., otic 

 vesicle; Neur. 4, Ncur. 5, neuromeres four and five of the myelencephalon. 



plate, the auditory placode (Fig. 382 A). These are developed, dorsal to the 

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

 meres. (Fig. 383). The placodes are invaginated to form hollow vesicles which 

 close in embryos of 2.5 to 3 



Wall of myelencephalon 



Endolymph duct 



Vestibular anlage- 



mm., but remain temporarily 

 attached to the ectoderm 

 (Fig. 382 B). 



The auditory vesicle, or 

 otocyst, when closed and de- 

 tached, is nearly spherical, 

 but approximately at the 

 point where it was attached 

 to the ectoderm a recess, the 

 ductus endolymphaticus, is 

 formed. The point of origin 

 of this recess is shifted later 

 from a dorsal to a mesial 

 position (Figs. 384 and 385 a). 

 The endolymph duct corres- 

 ponds to that of selachian 



fishes, which remains permanently open to the exterior. In man, its extremity 

 is closed and dilated to form the endolymphatic sac (Fig. 385 e). 



The differentiation of the auditory vesicle has been described by His, Jr., 

 and more recently by Streeter (Amer. Jour. Anat., vol. 6, 1906). In an em- 

 bryo of about 7 mm. the vesicle has elongated, its narrower ventral process con- 



2S 



Cochlear anlage 



Fig. 384. — Right half of a transverse section through 

 the hind-brain and otic vesicle, showing the position of the 

 endolymph duct. From a human embryo 6.9 mm. long 

 (His). 



