THE EAR. 141 



between the two layers of each fold, the septa thereby acqniidng 

 increased thickness. 



The first sep tpm which appears divides the vesicle into i ts 

 t wo m ain cavitie s, sacculus and utricnlns. 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, th e^ utrignln s ; and a lower 

 n nd nntpr pnrtinTij thf> sanrnilnR The septum is at first confined 

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

 and, growing inwards, se parates the two divisions almost com- 

 pletely fro m each othe r, n. vpry sTpall aperture of communication 

 ninur prrniHtinrr bnt-n-rrn them. 



From the utriculus, t he semic ircnlar canals are form ed. Each 

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

 shut ofi 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. 76, 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. 



Th eampuUee of thespTnifircnlar na rials ar" f-^rnif"^ 1nt°r than 

 the canals themselves, n ot as dilat ations of the can als, but b y 

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

 ca nals open into fL 



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

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

 adult. Fro m its upper an d hinder portions three small bulg ings 

 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. 



