34 THE FROG 



200. In the mesencephalon, the mesocosle consists of the 

 cavities of the optic lobes and the iter or aqueduct 

 into which they open. This passage connects the 

 diacoele with 



201. the fourth ventricle or myelocoele, already noted 

 (179). This cavity narrows into 



202. the central canal of the spinal cord. 



Remove the roof of the auditory capsule (225) and note: 



203. the membranous labyrinth of the inner ear. Un- 

 usually favorable dissections reveal 



204. the dorsal utriculus with its 



205. three semicircular canals, each with its ampulla, 



206. and the ventral sacculus, with 



207. a delicate tube, the ductus endolymphaticus, run- 

 ning dorsad from the cephalic margin. 



