130 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



shadows the anterior vertical semicircular canal (as) and a little 

 later a similar bulging more ventral, in position— the horizontal 

 canal (h.c). The lagena also is foreshadowed by a slight downward 

 bulging of the floor of the otocyst. 



With further development the posterior vertical canal rudiment 



appears also as a bulg- 

 ing of the otocyst wall 

 continuous with that 

 which will form the 

 anterior canal (Fig. 72, 

 C-F, p.v). The three 

 canal rudiments come 

 to project more and 

 more prominently, the 

 recess assumes the 

 tubular shape of the 

 endolymphatic duct 

 and the lower portion 

 of the otocyst (saccule) 

 with its projecting 

 lagenaf pocket and en- 

 dolymphatic duct be- 

 comes more sharply 

 marked off from the rest 

 of the otocyst (utricle). 

 The pouch -like rudi- 

 ments of the semicir- 

 cular canals, as they 

 come to project more 

 freely from the utricle, 

 assume a flattened form 

 and finally the central 

 portion of the wall on 

 each side bulges in- 

 wards and fuses with 

 that on the other. 



In this way the 

 central portion of the 

 cavity of each pouch 

 becomes obliterated 

 while the persisting 

 peripheral part takes the form of a curved tube — the definitive canal. 

 At first the space subtended by the canal is traversed by a continu- 

 ous septum formed out of the fused walls but this soon disappears 

 leaving the canal as a. freely projecting arch which opens into the 

 utricle at each end. The ampulla appears at an early stage as a 

 dilatation of the canal rudiment at one end. 



As will already have been gathered, the three canal rudiments do 



-p.v: 



Fig. 72. — Illustrating the development of the otocyst in the 

 Fowl. (A-G after Rothig and Brugseh, 1902 ; H after 

 Retzius.) 



A and B, early stages ; C, towards end of seventh day ; D, to- 

 wards end of eighth day ; E, live days ; ¥, towards end of ninth 

 day ; G, towards end of twelfth day ; H, adult. A is a view from 

 in front, B from behind, while C-H represent the left otocyst as 

 seen from the left side, a.v, anterior vertical canal ; h.c, horizontal 

 canal; lag, lagena; p.v, posterior vertical canal; r, recess; s.e, 

 endolymphatic duct. - 



