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. 
lic. 172. — Model of the auditory labyrinth of the 
right side of a chick embryo of 8 days and 17 
hours; external view. (After R6thig 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 eanal. C. 
a., Anterior semicircular canal. C. 1., Lateral semi- 
circular canal. C. p., Posterior semicircular canal. 
Sa.e., Endolymphatie sac. U., Utriculus. Other 
abbreviations as before. 
A day later (ig. 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. 
