494 



EMBRYOLOGY. 



Later this appendage of the labjnrinth (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 Uttle toward the brain (fig. 277 nh), and the concave side of which 



~rig. 277.— Cross section through the head of a Sheep emhryo 1*6 cm. long, in the region of the 

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

 of the sao ; on the left, one that is situated somewhat farther forward. After Boettcher. 



hrii Auditory nerve ; vb, -vertical semicircular canal ; gc, ganglion cochleare (spirale) ; dc, ductus 

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

 utriculus and sacculus ; rl, recessus labyrinthi ; nh, after-brain. 



lies in close contact with the previously mentioned ganglionic enlarge- 

 ment (go) 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 



