DEVELOPMENT OF THE SENSE ORGANS 



385 



to form hollow vesicles which close in the embryos of 2.5 to 3 mm., but 

 remain temporarily attached to the ectoderm (Fig. 382 B). 



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

 spherical, but approximately at the point where it was attached to the 



Ectoderm 



Wall of hind-brain 



Neur. j 



Neur. 



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; o.v., otic vesicle; Neur. 4, 

 Neur. 5, neuromeres. 



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 corresponds to that of selachian 

 fishes, which remains per- 

 manently open to the ex- 

 terior. In man, its ex- 

 tremity is closed and dilated 

 to form the endolymph sac 

 (Fig. 385 /) 



In an embryo of about 

 7 mm. the vesicle has 

 elongated, its narrower 

 ventral process constitut- 

 ing the anlage of the 

 cochlear duct (Fig. 385 a). 

 The wider, dorsal portion 

 of the otocyst is the vest- 

 ibular anlage, which shows 

 indications dorsally of the 

 developing semicircular 

 canals. These are formed 



Watt of myelencephalo 



Endolymph duct 



Vestibular anlagi 



Cochlear anlage 



FIG. 384. Right half of a transverse section through 

 the hind-brain and otic vesicles, showing the position 

 of the endolymph duct. From a 6.9 mm. human 

 embryo (His). 



in ii mm. embryos as two 

 pouches the anterior and posterior canals from a single pouch at the 

 dorsal border of the otocyst, the lateral canal later from a lateral out- 

 pocketing (Fig. 385 c). Centrally, the walls of these pouches flatten and 

 fuse to form epithelial plates. In the three plates thus produced canals 

 are left peripherally, communicating with the cavity of the vestibule. 



25 



