484 



DEVELOPMENT OF THE CHICK. 



The Ear. The ears arise during the second day as a 

 pair of epiblastic depressions at the sides of the hind-brain 

 (fig. 91, EI). These pits sink inwards and become converted 

 into the two auditory vesicles, the mouths of which close by 

 the end of the third day. 



These vesicles give rise to the membranous labyrinth. 

 By foldings of their walls the utriculus becomes constricted 



BS 



Fig. 92. Transverse section across the fore-brain and eye of a Chick 

 Embryo at the sixtieth hour of incubation. On the right side the 

 section passes through the optic stalk ; on the left side it passes 

 just behind the stalk, x 45. 



BF, cavity of fore-brain. BS, cavity of commencing cerebral hemisphere. 

 MX, maxillary arch. OC, inner wall of optic cup. OD, outer wall of optic 

 cup. c L, lens. OS, optic stalk. 



off from the sacculus. From the utriculus the semicircular 

 canals are formed ; from the sacculus the cochlea arises. 

 These changes take place between the fifth and eleventh 

 days. The entire epithelial lining of the labyrinth is thus 

 derived from the epiblast. 



The accessory structures of the ear are of very diverse 

 nature, and are too complex for description in this placa. 



