114 



THE THIRD DAT. 



[CHAP. 



are developed out of the mesoblastic investment of the 

 vesicle, the (lymphatic) space between the osseous and 

 membranous labyrinths being hollowed out of the mesoblast, 

 and becoming rilled with perilymph as it is formed. The chief 

 stages are as follows : — 



The form of the vesicle as seen in transverse sections 

 of the head from being oval becomes somewhat triangular 

 with the apex directed downwards (*. e. towards the ventral 

 surface of the body). (Fig. 32.) 



The apex continues to develope until it becomes a some- 

 what pointed process directed inwards (Fig. 34, (7(7), and lying 

 somewhat in front (nearer the face than) the rest of the vesicle, 



Fig. 35. 



Section of the Head of a Fcetal Sheep (20 mm. in length). 



(Copied from Eoettclier.) 



E.V. Recessus "Vestibuli. V. B. Vertical semicircular canal. II. B. Horizontal 

 semicircular canal. CC. Cochlear canal. 0. Cochlear eran<rlion. 



