294 



THE DEVELOPMENT OF THE CHICK 



cochlearis; the tip of the lagena in its growth ventralward has 

 reached the horizontal level of the notochord. The sacculus is 

 barely indicated yet, but is clearly seen on the seventh day as 

 a slight protuberance on the median surface of the uppermost 

 part of the pars inferior; it lies in front of the lower end of the 

 endolymphatic duct at a slightly lower level and is separated by 

 two depressions above and below, from the anterior ampulla 

 and the ductus cochlearis respectively. The furrows above the 

 sacculus and below the ampulla of the frontal semicircular canal 

 mark the boundary between the pars superior and inferior. 



Fig. 172. — Model of the auditory lal)yrinth of the 



right side of a chick embryo of 8 days and 17 



hours ; external view. (After Rothig and Brugsch.) 



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

 A. 1., Ampulla of the lateral semicircular canal. A. 

 p., Ampulla of the posterior semicircular canal. C. 

 a., Anterior semicircular canal. C. 1., Lateral semi- 

 circular canal. C. p., Posterior semicircular canal. 

 Sa. e., Endolymphatic sac. U., Utriculus. Other 

 abbreviations as before. 



A day later (Fig. 172), these furrows have cut in deeper and 

 have become continuous on the median surface; the lagena has 

 enlarged distally, and the sacculus is a hemispherical protuber- 

 ance. The tip of the lagena lies beneath the hind-brain (Fig. 



