292 THE DEVELOPMENT OF THE CHICK 
the apex of the utriculus; and the horizontal canal formed from 
the external pocket extends between the separated lower ends 
of the other two. 
We must now proceed to a more detailed examination. In 
point of time the anterior (sagittal) 
semicircular canal is the first to be 
formed (Fig. 171); the external (hori- 
zontal or lateral) canal comes next, 
and considerably later the posterior 
(frontal). Thus the anterior canal 
is at first the largest, the external 
next, and the posterior the smallest. 
These differences however, 
largely compensated in the course 
of the embryonic development. The 
ampullze appear as dilations in the 
pockets even before the canals are 
are, 
Fic. 170. — Model of the auditory 
labyrinth of a chick embryo of 6 
days and 17 hours; external view. 
(After R6éthig and Brugsch.) 
formed, and are conspicuous dila- 
tions by the time that the central 
parts of the pockets have broken 
through (Fig. 172). 
C. a., Poeket for formation of 
anterior semicircular canal. C. 1., 
Pocket for formation of lateral 
Figs. 170-173 show the pockets 
a and canals at six days seventeen 
semicircular canal. C. p., Pocket ; 
for formation of posterior semicir- hours, seven days seventeen hours, 
cular canal. D. ¢., Duetus eoch- 
learis. D.e., Endolymphatie duct. 
La., Lagena. 
eight days seventeen hours, and 
eleven days seventeen hours. It 
will be seen that, whereas the an- 
terior and lateral canals are formed from the start in planes at 
right angles to one another, viz., the sagittal and horizontal, the 
posterior canal is not at first in the third or frontal plane, but 
gradually assumes it. 
The form of the utriculus is gradually assumed during the 
formation of the semicircular canals; it becomes drawn out into 
a roughly triradiate form, so that it consists of a central cavity 
and three sinuses, viz., the median sinus which receives the end 
of the anterior and posterior semicircular canals, the posterior 
sinus situated above the ampulla of the external semicircular 
canal, and the anterior sinus in the region of the ampull of the 
horizontal and sagittal semicireular canals (ef. Fig. 173). A short 
distance in front of the posterior sinus on the median face of 
