124 



THE THIRD DAY. 



[chap 



AOA 



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

 OF THE Third Day of Incubation. 



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

 thicker sides of the ventricle. 



Ch. Notoohord — (diagrammatic shading). 



CV. Anterior cardinal or jugular vein. 



CC. Involuted auditory vesicle. CO points to the end which 

 will form the cochlear canal. RL. Eecessus labyrinthi. /ly. 

 hypoblast lining the alimentary canal. Ay is itself placed in 

 the cavity of the alimentary canal, in that part of the canal 

 which will become the throat. The ventral (anterior) wall of 

 the canal is not shewn in the section, but on each side are 

 seen portions of a pair of visceral arches. In each arch 

 is seen the section of the aortic arch A OA belonging to the 

 visceral arch. The vessel thus cut through is running 

 upwards towards the head, being about to join the dorsal 

 aorta AO. Had the section been nearer the head, and 

 carried through the plane at which the aortic arch curves 



