THE EAR. 141 



between the two^ layers of each fold, the septa thereby acquiring 

 increased thickness. 



The first septum which appears divides the vesicle into its 

 two main cavities, sacculus and utriculus. It arises in tadpoles 

 of about 11 mm. length as a fold of the outer wall of the vesicle, 

 which projects somewhat obliquely across the cavity, dividing it 

 into an upper and inner division, the utriculus ; and a lower 

 and outer portion, the sacculus. The septum is at first confined 

 to the hinder part of the vesicle, but soon extends all round it ; 

 and, growing inwards, separates the two divisions almost com- 

 pletely from each other, a very small aperture of communication 

 alone persisting between them. 



From the utriculus, the semicircular canals are formed. Each 

 canal is really a portion of the utriculus, which becomes partially 

 shut off from the main cavity by the formation of a septum 

 along the middle portion of its length ; remaining, however, in 

 communication with the cavity at each end. Each septum is 

 formed by two separate folds, which grow towards each other 

 from opposite sides of the vesicle, meet along their edges, and 

 fuse to complete the septum (Fig. 75, p. 162). The septum 

 soon thickens, through the ingrowth of mesoblast between its 

 layers ; it also elongates, and so causes lengthening of the canal, 

 which gradually acquires the adult shape and relations. 



Of the three semicircular canals, the anterior vertical and 

 the horizontal are formed simultaneously, and first appear in 

 tadpoles of about 11 mm. length. The posterior vertical canal 

 arises in the same way, but at a slightly later stage, in tadpoles 

 of about 15 mm. length. 



The ampullas of the semicircular canals are formed later than 

 the canals themselves, not as dilatations of the canals, but by 

 constriction of parts of the utriculus, at the places where the 

 canals open into it. 



The second division of the vesicle, or sacculus, grows down- 

 wards, and soon acquires the pouch-like character it has in the 

 adult. From its upper and hinder portions three smallbulgings 

 or pouch-like outgrowths appear, which together form the 

 cochlea. Of these, the lagena cochleae is the largest and the 

 earliest to appear, arising in tadpoles of about 15 mm. length ; 

 the pars neglecta appears shortly afterwards, and the pars 

 basilaris last of all. 



