712 MANUAL OF PHYSIOLOGY. 



the corneal tissue proper. The epiblast forms the epithelial or 

 conjunctival covering of the eyeball. 



The involution of mesoblast through the choroidal fissure, 

 which forms the vitreous humor, indents the optic stalk, and 

 forms the central artery of the retina. The choroidal fissure is 

 gradually obliterated, and its position may sometimes be marked 

 by a permanent fissure in the iris (coloboma iridis). 



The rudimentary lens is a spherical body, hollow in the centre, 

 made up of an anterior and posterior wall, each of which is formed 



FIG. 292. 



Section through the head of a chick at the third day, showing the origin 

 of the lens. a. Epiblast thickened at c, which is the point of origin of the 

 lens. o. Optic vesicle. T^. Anterior cerebral vesicle. V v Posterior 

 cerebral vesicle. 



of columnar cells. The posterior wall of the lens increases 

 greatly in thickness, and approaching the anterior wall obliter- 

 ates the original cavity of the lens. 



The cells forming this wall become very much elongated, and 

 form long fibre-like columnar cells. The cells of the anterior 

 wall, from being a columnar epithelium, are modified to a flat- 

 tened epithelium, and finally become the layer of epithelium 

 lining the anterior surface of the capsule of the lens. The cap- 



