lU 



THE THIRD DAY. 



[chap. 



are developed out of tlie mesoblastic investment of the 

 vesicle, the (lymphatic) space between the osseous and 

 membranous labyriuths being hollowed out of the mesoblast, 

 and becoming filled 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 [i. 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, CG), and lying 

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



Fig. 35. 



Section of the Head or a Fcetal Sheep (20 aim. in length), 

 (Copied from Boettoher.) 

 R. V. Eecessus Veatibuli. V.B. Vertical semicircular canal. II.B. Horizontal 

 semicircular canal. C.C. Cochlear canal. G. Cochlear ganglion. 



