I EARING 



1031 



itself, becomes the cochlear duct, the vestibular extremity of which is subsequently 

 constricted to form the canalis reuniens. Three others appear as disk-like evagi- 

 nations on the surface of the vesicle; the central parts of the walls of the disks 

 coalesce and disappear, while the peripheral portions persist to form the semi- 



Endolymph Endolymph 



Vestib. pouchy ^^ Vestib. pouch... 



Coch. pouch ^ 



6.6 mm. median 



Cochlea 1 



Utric.-sacc. 



9 mm. median 



11 mm. median 



Reuniens 





20 mm. median so mm , median 



FIG. 901. Median views of membranous labyrinth and acoustic complex in human embryos. X 25 dia. (Streeter.) 



circular ducts; of these the superior is the first and the lateral the last to be com- 

 pleted (Fig. 902) . The central part of the vesicle represents the membranous 

 vestibule, and is subdivided by a constriction into a smaller ventral part, the 

 saccule, and a larger dorsal and posterior part, the utricle. This subdivision is 



