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 (ti) by the resorption of the fused epithelial 

 areas, whereas at the second place {vb') 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, 



Fig. 279. — View produced by com1)inatioii from two cross sections througll the labyrinth of a 



Sheep embryo 2*8 cm. long, after Boettcher. 

 rl, Recessufl labyrinthi ; rl*, its flask -like enlargement ; r6, hb, vertical and horizontal canals ; 



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



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



cochlta ; sp, 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. 



