494 



EMBRYOLOGY. 



Later this appendage of the labyrinth (figs. 276-9 rl) grows out 

 dorsally to a great length, during which its walls come into close 

 contact with each other, excepting at the blind end, which is enlarged 

 into a small sac (fig. 279 rl *). 



Meanwhile the auditory sac itself (figs. 275-7) begins to be 

 elongated and to be formed into a ventrally directed conical process 

 (dc), the first fundament of the ductus cochlearis, which is curved inward 

 a little toward the brain (fig. 277 nh), and the concave 'side of which 



Pig. 277. Cross section through the head of a Sheep embryo 1'6 cm. long, in the region of the 

 labyrinth-sac. On the right side is represented a section which passes through the middle 

 of the sac ; on the left, one that is situated somewhat farther forward. After BOETTCHEB. 



hn, Auditory nerve ; vb, vertical semicircular caual ; gc, ganglion cochleare (spirale) ; dc, ductus 

 cochlearis ; /, inward-projecting fold, whereby the sac of the labyrinth is divided into 

 utriculus and sacculus ; rl, reeessus labyrinth! ; nil, after-brain. 



lies in close contact with the previously mentioned ganglionic enlarge- 

 ment (gc) of the auditory nerve (hn). 



It will be serviceable in the following description if we now 

 distinguish an upper and a lower division of the labyrinth. They are 

 not yet, it is true, distinctly delimited from each other, but in later 

 stages they become more sharply separated by an inward-projecting 

 fold (figs. 277-9/). 



The upper part (pars superior) furnishes the utriculus and the 

 semicircular canals. Of the latter the two vertical canals arise first, 

 the horizontal canal being formed later. The method of their origin 



