VI.] 



THE EAR. 



157 



l.aa. aortic trunk of mandibular arch ; pp. head cavity of 

 mandibular arob ; Ivc. alimentary pouch which will form the 

 first visceral cleft ; Th. rudiment of thyroid body. 



canal (Fig. 27, au. p.). It then had the form of a 

 shallow pit with a widely open mouth, similar in form 

 to that shewn for an embryo dog-fish in Fig. 53, aw. p. 

 Before the end of the third day, its mouth closes up and 

 all signs of the opening are obliterated. The pit thus 

 becomes converted into a closed vesicle, lined with 

 /epiblast, and surrounded by mesoblast. This vesicle is 

 the otic vesicle, whose cavity rapidly enlarges while its 

 walls become thickened (Fig. 54, GC). 



cv 



AOA 



Section through the Hind-Brain of a Chick at the end 

 OF the Third Day of Incubation. 



IT. Fourth ventricle. The section shews the very thin roof and 

 thicker sides of the ventricle. 



