AVES. 



177 



third ventricle, the iter (with a prolongation into the optic lobe 

 on each side), and the fourth ventricle. 



By the third day the lens of the eye has become formed by 

 an invagination of the epiblast, and other changes in the eye 

 have taken place. The external opening of the auditory pit is 

 closed before the completion of the third day (fig. 114, RL) ; 

 and the rudiments of the external parts of the organ of smell 

 have become formed as small pits on the under surface of the 

 fore-brain (fig. 112, N). Like the lens and the labyrinth of the 

 ear, they are formed as invaginations of the external epiblast ; 

 unlike them they are never closed up. 



During the second and third days there are formed the 

 visceral or branchial clefts, homologous with those of the 



cv 



cc 



FIG. 114. SECTION THROUGH THE HIND-BRAIN OF 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. Notochord; CV. Anterior cardinal vein; CC. Involuted auditory 

 vesicle ; CC points to the end which will form the cochlear canal ; RL . Recessus 

 labyrinth! (remains of passage connecting the vesicle with the exterior); hy. Hypoblast 

 lining the alimentary canal; AO., AOA. Aorta, and aortic arch. 



Ichthyopsida, though never developing branchial processes from 

 their walls. 



They are however real clefts or slits passing right through 

 the walls of the throat, and are placed in series on either side 

 B. in. 12 



