DEVELOPMENT OF THE SKULL OF EMYS 717 



high and vertical in position. Dorsally the cavum saccuH stands 

 in wide open connection with that of the vestibulum. Its boun- 

 daries are the capsular walls laterally, medially, posteriorly, and 

 anteriorly; and ventrally the basal plate which bulges ventrally 

 as already noted because of the great extension of the lagena in 

 a ventral direction. In its ventral portion the cavum sacculi 

 exhibits an antero-lateral and a postero-medial lobe which stand 

 in wide open communication with each other; the boundary 

 between the two extending from the median wall at the level of 

 the foramen acusticum posterius in a posterolateral direction. 

 The antero-lateral lobe extends on the outer aspect posteriorly 

 beyond the fenestra vestibuh which interrupts its lateral wall; on 

 its median aspect it extends posteriorly only as far as the foramen 

 acusticum posterius. In it is situated the sacculus. The postero- 

 medial lobe is somewhat deeper than the previous one so that its 

 ventral end projects shghtly below the level of the lateral lobe. 

 It is somewhat triangular in form with the posterior side in a 

 vertical plane. This encloses the lagena, the ductus perilympha- 

 ticus and the n. glossopharyngeus in its course between the exter- 

 nal and internal foramina of that nerve. Accordingly this cavity 

 exhibits two small but very important extensions; namely, the 

 sulcus glossopharyngeus (s.g.) and the canahs perilymphaticus 

 (c.per.). The sulcus glossopharyngeus is situated on the posterior 

 cochlear wall ventral to the cavum ampullare posterius and 

 extends transversely from the foramen glossopharyngei inter- 

 num in a lateral and ventral direction to the foramen glosso- 

 pharyngei externum. It is in the form of a cyhndrical groove of 

 uniform size which opens into the cavum sacculi as already indi- 

 cated. Near its median extremity it intersects the ventral wall 

 of the cavum ampullare posterius so that it shows communica- 

 tion with this part as well. It is completely filled by the nerve 

 which thus comes to He with its anterior half projecting freely 

 into the cavum sacculi. The canalis perilymphaticus extends 

 in an anterior direction from the fenestra cochleae horizontally 

 through the posterior cochlear wall. It opens into the sacculus 

 at a point half way between the dorsal and ventral ends of that 

 part and somewhat medial to the middle plane of the cavity. • 



