ORGANS OF SPECIAL SENSE 293 
the utriculus 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) Development of the Pars Inferior Labyrinthi; 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 
Tic. 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 Réthig and Brugsch.) 
A.a., Ampulla of the anterior semicircular canal. A. p., Ampulla 
of the posterior semicircular canal. C. a., Anterior semicircular 
eanal. C.1., Pocket for formation of the lateral semicircular canal. 
C. p., Pocket for formation of the posterior semicircular canal. Sa., 
Saeculus. 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 
