496 EMBRYOLOGY. 



The walls of this large pocket come into contact with each other and 

 fuse at two different places. At one of them there has already 

 been formed, in the preparation from which this model (fig. 276) was 

 constructed, an opening (tf) by the resorption of the fused epithelial 

 areas, whereas at the second place (vb 1 ) the epithelial membrane is 

 still preserved. Between the fused parts of the pocket there remains 

 open a middle region, which is indicated in the model by an asterisk, 



H- 

 Fig. 279. View produced by combination from two cross sections through the labyrinth of a 



Sheep embryo 2'8 cm. long, after BOETTCHER. 

 rl, Recessus labyrinthi ; rl*, its flask-like enlargement ; -cb, Tib, vertical and horizontal canals; 



U, utriculus ; S, sacculus ; /, fold by means of which the labyrinth is divided into sacculus 



and utriculus ; cr, canalis reunions ; dc, ductus cochlearis ; kk, cartilaginous capsule of the 



cochlea ; gp, sinus petrosus inferior. 



and this becomes the common arm (sinus superior) of the two vertical 

 canals. Thus embryology furnishes for this peculiarity, too, a simple 

 satisfactory explanation. 



That which remains of the upper portion of the auditory vesicle, 

 after the semicircular canals have grown forth from its wall, is 

 called the utriculus (figs. 278-80 U). 



Meanwhile no less significant and fundamental alterations take 

 place in the lower part of the auditory sac and lead to the formation 

 of sacculus and ductus cochlearis. 



