110 



THE THIRD DAY. 



[CHAP. 



epiblast furnishes the body of the cornea, the epiblast itself 

 remaining as the anterior corneal epithelium. 



A portion of mesoblast, carried in from the front by the 

 lens during its involution, gives rise to the capsule of the 

 lens and suspensory ligament, while some mesoblast entering 

 on the under side through the choroidal fissure becomes (in 

 birds) the pecten, and probably also contributes to the 

 vitreous humour. 



Of the walls of the optic cup, the thinner outer (posterior) 

 wall becomes, behind the line of the ora serrata, the pigment- 

 epithelium of the choroid, while the thicker inner (anterior) 

 wall supplies all the elements of the retina, including the 

 rods and cones which grow out from it into the pigment- 

 epithelium. 



FIG. 32. 



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. Notochord (diagrammatic shading). 

 CV. Anterior cardinal vein. 



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

 cochlear canal. ML. Kecessus labyrinthi. Jiy. Hypoblast lining the 

 alimentary canal. AO, AOA, Aorta, and aortic arch. 



