292 



THE DEVELOPMENT OF THE CHICK 



the apex of the utricukis; 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 are, however, 

 largely compensated in the course 

 of the embryonic development. The 

 ampullae appear as dilations in the 

 pockets even before the canals are 

 Pig. 170. — Model of the auditory formed, and are conspicuous dila- 

 labyrinth of a chick embryo of 6 tions by the time that the central 

 days and 17 hours; external view, parts of the pockets have broken 

 (After Rothig and Brugsch.) throuo'h (Fig. 172). 



C. a., Pocket for formation of „. ^^^ ^^„ , ,, _^^u^+^ 



anterior semicircular canal. C.I., FlgS. 1/0-1/3 show the pocketS 



Pocket for formation of lateral ^Tid canals at six days seventeen 

 semicircular canal. C. p., Pocket , , _+^^„ u^tt,.^ 



for formation of posteriir semicir- hours, seven days seventeen houis, 



cular canal. D. c, Ductus coch- eight days seventeen hours, and 



learis. D. e., Endolymphatic duct. . ^ , ■> ,^ j- 



La., LagenL 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 transverse 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 ampullae of the 

 horizontal and sagittal semicircular canals (cf. Fig. 173). A short 

 distance in front of the posterior sinus on the median face of 



