ORGANS OF SPECIAL SENSE 



293 



the utriciilus occur the openings of the ductus endolymphaticus, 

 sacculus, and ductus cochlearis; the two latter derived from the 

 pars inferior of the otocyst, to the development of which we 

 now turn. 



(c) Develop7nent of the Pars Inferior Lahyrinthi; Lagena, 

 Ductus Cochlearis, and Sacculus. During the changes described 

 in the pars superior labyrinthi, the pars inferior has developed 

 into the ductus cochlearis and lagena on the one hand, and the 

 sacculus on the other. Throughout the series of the vertebrates 



Fig. 171. — Model of the auditory labyrinth of the left side of a chick 



of 7 days and 17 hours. A. Median view. B. External view. 



(After Rothig and Brugsch.) 



A. a., Ampulla of the anterior semicircular canal. A. p., Ampulla 

 of the posterior semicircular canal. C. a., Anterior semicircular 

 canal. C. 1., Pocket for formation of the lateral semicircular canal. 

 C. p., Pocket for formation of the posterior semicircular canal. Sa., 

 Sacculus. Other abbreviations as, before. 



the structure of the pars superior is very uniform; the pars inferior, 

 on the other hand, has a characteristic structure in each class 

 and exhibits in general a progressive evolution. The condition 

 in the chick is characteristic on the whole for the class of birds. 

 At six days the lower division of the otocyst has grown out 

 ventralward into a deep pouch which is curved posteriorly and 

 towards the middle line (Fig. 170); the terminal portion is the 

 rudiment of the lagena, and the intermediate portion of the ductus 



