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 dose 

 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 



hn 

 vb 



dc 



Fig, 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 BOETTCHER. 



hn, 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, recessns labyrinth! ; tth, after-brain. 



lies in close contact with the previously mentioned ganglionic enlarge- 

 ment (yc) 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 

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

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



